In article , "Greg_False"
writes:

"Avery Fineman" wrote in message
...
In article , "Greg_False"

writes:

Thanks for the prompt and very helpful response.

I do not know enough about radio circuits to fiddle with installing
additional preamps directly into the radio it' self.

Okay. How old is the radio you want to use in the Land Rover?

For those made since around 1970 in HF use, the sensitivity
of the receiver section is already near optimum. Won't be
much improvement by adding an outboard preamp.

I want to improve my reception as I normally travel with a group of other
Land Rover owners who also have 29MH radios but they are not as fussy as I
am about their installations and in many cases they can hear me but I cannot
hear them.

Then I would look into the connections and possible corrosion
on your existing mobile antenna mounting. If you are traveling
with others no more than perhaps 10 miles or so distant, your
transmitter output may be enough, even though attenuated, to
break the others' squelch and they could hear you.

Chances are that the receiver of your transceiver is just off-tune
and some alignment is needed. I'd check that first, can be
done on-air, receive only with a friend's transmissions.

I'd begin with the mobile antenna mount and connection to the
coax, even the coax itself. Use an ohmmeter and check for
low but finite resistances there. Take apart the mobile antenna
mount and wire-brush (rotary on a drill is my recommendation)
the conductor surfaces. Some corroded contact surfaces may
look clean but an ohmmeter may show otherwise. If there are
lock-washers there, a dab of silicone grease (the thermal
conductivity grease used on power transistor mountings works)
will cut down on future oxidation. The teeth of lock-washers
assures a positive metal-to-metal contact even with grease.

I was thinking of using a double pole double throw relay.

No problem there.

I could fit a micro switch to the mic which activates before the press to
talk switch activates the transmitter?

I like Tim Wescott's suggestion of using a lower-power relay coil
and detecting the RF from the transmitter first...saves having to
run a line from the transceiver for the PTT coil energization...but
that has a danger in the build-up time of transmitter power output
being faster than the detector-relay switchover time. If so, the
preamp will be fried before the relay has time to switch over.

Nearly all PTT transceivers of any kind have a line already there
inside from the PTT switch. Bring that one out and get an RF
detector to measure the transmitter RF output. Accuracy not
needed here. Use an oscilloscope with a slow time base, say
around 0.1 Sec, with a DC input capability, and compare the
timing of the PTT line and the detected RF output. The PTT
line MUST operate FIRST. With at least 0.1 Sec lag of RF
output versus the PTT line, the relay idea has a chance to work.

To be honest, I would suspect you might have some
misalignment in the receiver front end that's giving the
problem of low sensitivity. An outboard receiver preamp may
not help that condition much.

"Greg_False" wrote in message
...
Hi,

I want to fit a receiving preamp to my 29MH mobile radio which runs a
50watt
linear amplifier. My plan is to fit the preamp at the base of the 102"
whip
and to switch it in and out of the co-ax circuit (when transmitting) by
means of relays activated by the mic press to talk button. Can I use any
12v relays, well shielded by copper boxes around them and what will happen
to my 50ohms impedance?

As you can see I need all the guidance I can get:)

Use one of these preamps. YOu will have to put it between the rig and amp
as they are only good for about 25 watts. They have the switching built in.

http://www.advancedreceiver.com/page7.html

Subject: Antenna Preamp
From: "Greg_False"
Date: 6/18/04 1:35 PM Central Daylight Time
Message-id:


I want to fit a receiving preamp to my 29MH mobile radio which runs a

50watt

linear amplifier. My plan is to fit the preamp at the base of the 102"

whip

and to switch it in and out of the co-ax circuit (when transmitting) by
means of relays activated by the mic press to talk button.

Can we presume the frequency as about 29 MHz?

What you propose is quite proper to do but that raises the question
of WHY the need to do it?

It would seem that it would be easier all around to add some
amplification in the radio's receiver, behind its built-in switching
or T/R circuitry. The amount of loss experienced in the 10m
band coax isn't going to affect much unless the mobile is a
double-trailered semi with antenna at the aft trailer and radio is
in the tractor's cab. That coax loss will be there as much on
transmit as it will be on receive...which is to say, very little.


Can I use any
12v relays, well shielded by copper boxes around them and what will

happen

to my 50ohms impedance?

Very little will happen to the 50 Ohm impedance if you use most
any common small relays that can handle 50 W of anything (with
enough fudge factors to withstand about 200 W of anything).

Use Ohm's Law of Resistance to find the RMS voltage and current
at 50 Ohms for 50 Watts. Multiply those by 1.414 to find the peaks.
Since the mobile antenna isn't a perfect 50 Ohms, multiply the
peaks by 4 as a safety margin. Those are the breakdown voltages
and current-carrying requirements of the relay contacts. Try to keep
the relay contact wiring to/from antenna and radio's coax as short as
possible.

It would be wise to use a "break-before-make" contact set on the
relays. That avoids having a "one-time-use" preamp circuit where
the transmit RF can wipe out the preamp input. Contact action
can be checked at DC and with a scope to be sure.

First of all, the typical (8 foot) whip isn't going to be at a common
impedance over the whole 10 m band. It's characteristics will
change between different vehicle shapes/conductive surfaces even
if at the same frequency in comparing vehicles. No matter, those
things work okay as they are...work better with some kind of
antenna matching network. Antenna pattern will vary depending
on the vehicle and its conductive surface shape and topography.
That is normal, nothing to be overly concerned about.

Only nit-pickers will get into hissy-fits on antenna impedance and
antenna patterns and so forth. :-)


As you can see I need all the guidance I can get:)

We can all help there although this thread might get into a different
direction with nit-picking that is sure to evolve...:-)

Relays are okay to use even if they have (what seems to be) "large"
contact arrangements (of around an inch or two). Some will insist
that such things will mess up the 50 Ohm impedance (of the coax)
but then they probably haven't measured actual mobile antenna
installations in detail, such as the connection between coax and
antenna base. No matter, any relay that can handle 200 W of
anything has contact structures that are about 0.01 wavelength
at 10 m and won't affect the characteristic impedance very much.

What I see as a potential problem is the relay contact making
the transmitter output directly into the preamp input, even for a
few milliseconds. That's blow-out time for the preamp with 50 W
potential output...50 W in a 50 Ohm system equals 50 V RMS
or 71 V peak, current is 1 A RMS or 1.4 A peak. [P = E^2 / R
and = I^2 x R] Relay contacts can operate in about 5mS (reed)
to 10mS (rotary spider-armature) to 100 mS (larger leaf-type
contact stacks). It may be that the transmitter output may not
reach full output in a short time (most probable) but that is an
unknown here.

A protective box for the relay is very good for environmental
protection (think salty water in northern climes in winter) but it
won't help otherwise for RF in this case. A totally-enclosing
ABS plastic box should be fine.


Putting on the skeptic's hat, I'd have to ask why the need of a
preamp? If more receive gain is desired, the safer place to do
that could be in the transceiver itself. The T/R switching is
already
there and presumably safe to use as it is.


retired (from regular hours) electronic engineer person



Hi,

Thanks for the prompt and very helpful response.

I do not know enough about radio circuits to fiddle with installing
additional preamps directly into the radio it' self.

I want to improve my reception as I normally travel with a group of
other
Land Rover owners who also have 29MH radios but they are not as fussy as
I
am about their installations and in many cases they can hear me but I
cannot
hear them.

I was thinking of using a double pole double throw relay.

I could fit a micro switch to the mic which activates before the press
to
talk switch activates the transmitter?

Thanks again
Greg




Even at 10 meters a modern radio should be fine, unless you have an
older rig or one that's got a reputation for being crappy I'd suspect a
fault with the radio or the antenna installation. I'd want to do a bit
of troubleshooting before I went off building modifications to something
that may just need repair.

At lower HF frequencies you can test your receiver noise level pretty
quickly by attaching an antenna -- if the atmospheric noise is
discernible then you have no trouble. I honestly don't know if this is
still a valid test at 10M.

If you can, see if you can swap rigs with somebody temporarily and go
driving. If the problem follows the rig then you have rig problems, if
it follows the car you have car problems. If you have no problem with
your car shut down but the noise level goes up with it running
(particularly if you hear the ignition) then you have ignition noise
problems. You may have an antenna that's more of a dummy load than a
radiator (and receiver).

If you still think you must have a preamp you should have a better time
installing the preamp inside the radio than outside -- there will be a
spot in the radio that comes after the T/R switch but before the mixer.
If the radio already has a preamp then you probably have one of the
aforementioned faults, and should be looking at "fix" rather than
"modify".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Hi Tim,

I don't think there's anything wrong with the radio, it is fairly new, I was
hoping to snatch the last bit of a fading signal from the ether.

It is a very simple radio, the only controls are channel selection, squelch
and volume.

I know, get a more professional bit of kit, but I'm always challenged to try
and improve things, which eventually costs more than buying the correct
thing in the first place.

I thing I've got the message, don't waste my time:((

Thanks to all for the help.

Greg













Nothing to add ... I just wanted to make this quote longer.

:^]

jw
K9RZZ

On 18 Jun 2004 23:15:01 GMT, (J999w) wrote:

Subject: Antenna Preamp
From: "Greg_False"
Date: 6/18/04 1:35 PM Central Daylight Time
Message-id:


I want to fit a receiving preamp to my 29MH mobile radio which runs a

50watt

linear amplifier. My plan is to fit the preamp at the base of the 102"

whip

and to switch it in and out of the co-ax circuit (when transmitting) by
means of relays activated by the mic press to talk button.

Can we presume the frequency as about 29 MHz?

What you propose is quite proper to do but that raises the question
of WHY the need to do it?

It would seem that it would be easier all around to add some
amplification in the radio's receiver, behind its built-in switching
or T/R circuitry. The amount of loss experienced in the 10m
band coax isn't going to affect much unless the mobile is a
double-trailered semi with antenna at the aft trailer and radio is
in the tractor's cab. That coax loss will be there as much on
transmit as it will be on receive...which is to say, very little.


Can I use any
12v relays, well shielded by copper boxes around them and what will

happen

to my 50ohms impedance?

Very little will happen to the 50 Ohm impedance if you use most
any common small relays that can handle 50 W of anything (with
enough fudge factors to withstand about 200 W of anything).

Use Ohm's Law of Resistance to find the RMS voltage and current
at 50 Ohms for 50 Watts. Multiply those by 1.414 to find the peaks.
Since the mobile antenna isn't a perfect 50 Ohms, multiply the
peaks by 4 as a safety margin. Those are the breakdown voltages
and current-carrying requirements of the relay contacts. Try to keep
the relay contact wiring to/from antenna and radio's coax as short as
possible.

It would be wise to use a "break-before-make" contact set on the
relays. That avoids having a "one-time-use" preamp circuit where
the transmit RF can wipe out the preamp input. Contact action
can be checked at DC and with a scope to be sure.

First of all, the typical (8 foot) whip isn't going to be at a common
impedance over the whole 10 m band. It's characteristics will
change between different vehicle shapes/conductive surfaces even
if at the same frequency in comparing vehicles. No matter, those
things work okay as they are...work better with some kind of
antenna matching network. Antenna pattern will vary depending
on the vehicle and its conductive surface shape and topography.
That is normal, nothing to be overly concerned about.

Only nit-pickers will get into hissy-fits on antenna impedance and
antenna patterns and so forth. :-)


As you can see I need all the guidance I can get:)

We can all help there although this thread might get into a different
direction with nit-picking that is sure to evolve...:-)

Relays are okay to use even if they have (what seems to be) "large"
contact arrangements (of around an inch or two). Some will insist
that such things will mess up the 50 Ohm impedance (of the coax)
but then they probably haven't measured actual mobile antenna
installations in detail, such as the connection between coax and
antenna base. No matter, any relay that can handle 200 W of
anything has contact structures that are about 0.01 wavelength
at 10 m and won't affect the characteristic impedance very much.

What I see as a potential problem is the relay contact making
the transmitter output directly into the preamp input, even for a
few milliseconds. That's blow-out time for the preamp with 50 W
potential output...50 W in a 50 Ohm system equals 50 V RMS
or 71 V peak, current is 1 A RMS or 1.4 A peak. [P = E^2 / R
and = I^2 x R] Relay contacts can operate in about 5mS (reed)
to 10mS (rotary spider-armature) to 100 mS (larger leaf-type
contact stacks). It may be that the transmitter output may not
reach full output in a short time (most probable) but that is an
unknown here.

A protective box for the relay is very good for environmental
protection (think salty water in northern climes in winter) but it
won't help otherwise for RF in this case. A totally-enclosing
ABS plastic box should be fine.


Putting on the skeptic's hat, I'd have to ask why the need of a
preamp? If more receive gain is desired, the safer place to do
that could be in the transceiver itself. The T/R switching is
already
there and presumably safe to use as it is.


retired (from regular hours) electronic engineer person



Hi,

Thanks for the prompt and very helpful response.

I do not know enough about radio circuits to fiddle with installing
additional preamps directly into the radio it' self.

I want to improve my reception as I normally travel with a group of
other
Land Rover owners who also have 29MH radios but they are not as fussy as
I
am about their installations and in many cases they can hear me but I
cannot
hear them.

I was thinking of using a double pole double throw relay.

I could fit a micro switch to the mic which activates before the press
to
talk switch activates the transmitter?

Thanks again
Greg




Even at 10 meters a modern radio should be fine, unless you have an
older rig or one that's got a reputation for being crappy I'd suspect a
fault with the radio or the antenna installation. I'd want to do a bit
of troubleshooting before I went off building modifications to something
that may just need repair.

At lower HF frequencies you can test your receiver noise level pretty
quickly by attaching an antenna -- if the atmospheric noise is
discernible then you have no trouble. I honestly don't know if this is
still a valid test at 10M.

If you can, see if you can swap rigs with somebody temporarily and go
driving. If the problem follows the rig then you have rig problems, if
it follows the car you have car problems. If you have no problem with
your car shut down but the noise level goes up with it running
(particularly if you hear the ignition) then you have ignition noise
problems. You may have an antenna that's more of a dummy load than a
radiator (and receiver).

If you still think you must have a preamp you should have a better time
installing the preamp inside the radio than outside -- there will be a
spot in the radio that comes after the T/R switch but before the mixer.
If the radio already has a preamp then you probably have one of the
aforementioned faults, and should be looking at "fix" rather than
"modify".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Hi Tim,

I don't think there's anything wrong with the radio, it is fairly new, I was
hoping to snatch the last bit of a fading signal from the ether.

It is a very simple radio, the only controls are channel selection, squelch
and volume.

I know, get a more professional bit of kit, but I'm always challenged to try
and improve things, which eventually costs more than buying the correct
thing in the first place.

I thing I've got the message, don't waste my time:((

Thanks to all for the help.

Greg













Nothing to add ... I just wanted to make this quote longer.

:^]

jw
K9RZZ

it is a lot to add for those who has some technical experience, but it
seems not to be important in this discussion

---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

"Greg_False" wrote in message
...

I don't think there's anything wrong with the radio, it is fairly new, I
was
hoping to snatch the last bit of a fading signal from the ether.

It is a very simple radio, the only controls are channel selection,
squelch
and volume.

I know, get a more professional bit of kit, but I'm always challenged to
try
and improve things, which eventually costs more than buying the correct
thing in the first place.

I thing I've got the message, don't waste my time:((

Greg

Amidst all the quotes and suggestions, I didn't see an explanation of why
people are saying what they are saying. Here's an attempt at an
explanation.

There is noise in the air, and more noise at lower frequencies. Meanwhile,
especially with transistors, it is easier to get gain at lower frequencies
than at higher. Once you can hear the noise, no additional gain is going to
help. The signal that was below the noise will still be below the noise,
only the noise and the signal will be louder.

At 40 meters, there is so much noise that it borders on impossible to make a
receiver that doesn't have more than enough gain. At 2 meters and above,
the noise is very low, and gain is hard to get. So hard, in fact, that
noise in the transistors becomes an issue. It isn't an issue at lower
frequencies. At VHF and above, if you want to increase the gain you need to
do it at the first stage, otherwise you will be amplifying the noise in the
earlier stages. Furthermore, at VHF and above, coax losses tend to be high,
so it makes sense to amplify the signal before it has been attenuated by the
coax, and possibly fallen into the internal noise of the amplifier. At HF,
coax loss isn't much of an issue (except for antennas operated at very high
SWR), so if you would need a preamp, putting it at the antenna end of the
coax isn't the big advantage that it is at VHF/UHF.

Now, the reason for a little waffling is that 10 meters is kind of in
no-man's land. Band noise is quite low, and it does take some work to get
the gain you need. It tends not to be expensive to get that gain, so even
cheap radios will sometimes have enough gain, but not always. Back thirty
years ago, it was hard to get enough gain on 10, so many older radios could
use a little help. But today, two bucks will buy a chip with ten times the
gain you can use on ten meters.

The reason for the suggestion on unplugging the antenna should now be
obvious. If you can hear the difference in background noise with the
antenna plugged in versus not, then you have enough gain in your receiver
that a preamp will do no good. This is a worthwhile experiment, given that
you have indicated it is a low end rig. I'm sure you hear "band noise", but
if it's a sufficiently low end rig that noise could be coming from inside
the receiver. Not likely, but still possible. The antenna experiment will
put that question to rest.

The reason for the idea of going into the radio to locate the preamp was
simply one of convenience - the relay you need is already there. Unlike at
VHF, there really isn't a big advantage to putting the preamp at the end of
the coax, so why not mount it near the radio where you already have power
and don't have to waterproof it, and taking it a step further, why not take
advantage of the relay that's already there.

Oh yes, and if you do build a preamp, don't sweat the SWR bump at the relay.
As long as you keep all the connections reasonably short and straight it
won't be an issue.

Hope this helps a bit

...

J999w wrote:
Subject: Antenna Preamp
From: "Greg_False"
Date: 6/18/04 1:35 PM Central Daylight Time
Message-id:

I want to fit a receiving preamp to my 29MH mobile radio which runs a

50watt


linear amplifier. My plan is to fit the preamp at the base of the 102"

whip


and to switch it in and out of the co-ax circuit (when transmitting) by
means of relays activated by the mic press to talk button.

Can we presume the frequency as about 29 MHz?

What you propose is quite proper to do but that raises the question
of WHY the need to do it?

It would seem that it would be easier all around to add some
amplification in the radio's receiver, behind its built-in switching
or T/R circuitry. The amount of loss experienced in the 10m
band coax isn't going to affect much unless the mobile is a
double-trailered semi with antenna at the aft trailer and radio is
in the tractor's cab. That coax loss will be there as much on
transmit as it will be on receive...which is to say, very little.



Can I use any
12v relays, well shielded by copper boxes around them and what will

happen


to my 50ohms impedance?

Very little will happen to the 50 Ohm impedance if you use most
any common small relays that can handle 50 W of anything (with
enough fudge factors to withstand about 200 W of anything).

Use Ohm's Law of Resistance to find the RMS voltage and current
at 50 Ohms for 50 Watts. Multiply those by 1.414 to find the peaks.
Since the mobile antenna isn't a perfect 50 Ohms, multiply the
peaks by 4 as a safety margin. Those are the breakdown voltages
and current-carrying requirements of the relay contacts. Try to keep
the relay contact wiring to/from antenna and radio's coax as short as
possible.

It would be wise to use a "break-before-make" contact set on the
relays. That avoids having a "one-time-use" preamp circuit where
the transmit RF can wipe out the preamp input. Contact action
can be checked at DC and with a scope to be sure.

First of all, the typical (8 foot) whip isn't going to be at a common
impedance over the whole 10 m band. It's characteristics will
change between different vehicle shapes/conductive surfaces even
if at the same frequency in comparing vehicles. No matter, those
things work okay as they are...work better with some kind of
antenna matching network. Antenna pattern will vary depending
on the vehicle and its conductive surface shape and topography.
That is normal, nothing to be overly concerned about.

Only nit-pickers will get into hissy-fits on antenna impedance and
antenna patterns and so forth. :-)



As you can see I need all the guidance I can get:)

We can all help there although this thread might get into a different
direction with nit-picking that is sure to evolve...:-)

Relays are okay to use even if they have (what seems to be) "large"
contact arrangements (of around an inch or two). Some will insist
that such things will mess up the 50 Ohm impedance (of the coax)
but then they probably haven't measured actual mobile antenna
installations in detail, such as the connection between coax and
antenna base. No matter, any relay that can handle 200 W of
anything has contact structures that are about 0.01 wavelength
at 10 m and won't affect the characteristic impedance very much.

What I see as a potential problem is the relay contact making
the transmitter output directly into the preamp input, even for a
few milliseconds. That's blow-out time for the preamp with 50 W
potential output...50 W in a 50 Ohm system equals 50 V RMS
or 71 V peak, current is 1 A RMS or 1.4 A peak. [P = E^2 / R
and = I^2 x R] Relay contacts can operate in about 5mS (reed)
to 10mS (rotary spider-armature) to 100 mS (larger leaf-type
contact stacks). It may be that the transmitter output may not
reach full output in a short time (most probable) but that is an
unknown here.

A protective box for the relay is very good for environmental
protection (think salty water in northern climes in winter) but it
won't help otherwise for RF in this case. A totally-enclosing
ABS plastic box should be fine.


Putting on the skeptic's hat, I'd have to ask why the need of a
preamp? If more receive gain is desired, the safer place to do
that could be in the transceiver itself. The T/R switching is

already

there and presumably safe to use as it is.


retired (from regular hours) electronic engineer person



Hi,

Thanks for the prompt and very helpful response.

I do not know enough about radio circuits to fiddle with installing
additional preamps directly into the radio it' self.

I want to improve my reception as I normally travel with a group of

other

Land Rover owners who also have 29MH radios but they are not as fussy as

I

am about their installations and in many cases they can hear me but I

cannot

hear them.

I was thinking of using a double pole double throw relay.

I could fit a micro switch to the mic which activates before the press

to

talk switch activates the transmitter?

Thanks again
Greg




Even at 10 meters a modern radio should be fine, unless you have an
older rig or one that's got a reputation for being crappy I'd suspect a
fault with the radio or the antenna installation. I'd want to do a bit
of troubleshooting before I went off building modifications to something
that may just need repair.

At lower HF frequencies you can test your receiver noise level pretty
quickly by attaching an antenna -- if the atmospheric noise is
discernible then you have no trouble. I honestly don't know if this is
still a valid test at 10M.

If you can, see if you can swap rigs with somebody temporarily and go
driving. If the problem follows the rig then you have rig problems, if
it follows the car you have car problems. If you have no problem with
your car shut down but the noise level goes up with it running
(particularly if you hear the ignition) then you have ignition noise
problems. You may have an antenna that's more of a dummy load than a
radiator (and receiver).

If you still think you must have a preamp you should have a better time
installing the preamp inside the radio than outside -- there will be a
spot in the radio that comes after the T/R switch but before the mixer.
If the radio already has a preamp then you probably have one of the
aforementioned faults, and should be looking at "fix" rather than

"modify".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Hi Tim,

I don't think there's anything wrong with the radio, it is fairly new, I was
hoping to snatch the last bit of a fading signal from the ether.

It is a very simple radio, the only controls are channel selection, squelch
and volume.

I know, get a more professional bit of kit, but I'm always challenged to try
and improve things, which eventually costs more than buying the correct
thing in the first place.

I thing I've got the message, don't waste my time:((

Thanks to all for the help.

Greg














Nothing to add ... I just wanted to make this quote longer.

:^]

jw
K9RZZ


Well, you could have done that better by adding more than a one line
comment! I mean really. Now go away, think about what you did, and come
back and post at least 4 lines of sentence to add to the thread. Then
you'll be doing it right! 8^)

- Mike KB3EIA -

Subject: Antenna Preamp
From: Mike Coslo
Date: 6/19/04 12:45 PM Central Daylight Time
Message-id:



J999w wrote:
Subject: Antenna Preamp
From: "Greg_False"
Date: 6/18/04 1:35 PM Central Daylight Time
Message-id:

I want to fit a receiving preamp to my 29MH mobile radio which runs a

50watt


linear amplifier. My plan is to fit the preamp at the base of the 102"

whip


and to switch it in and out of the co-ax circuit (when transmitting) by
means of relays activated by the mic press to talk button.

Can we presume the frequency as about 29 MHz?

What you propose is quite proper to do but that raises the question
of WHY the need to do it?

It would seem that it would be easier all around to add some
amplification in the radio's receiver, behind its built-in switching
or T/R circuitry. The amount of loss experienced in the 10m
band coax isn't going to affect much unless the mobile is a
double-trailered semi with antenna at the aft trailer and radio is
in the tractor's cab. That coax loss will be there as much on
transmit as it will be on receive...which is to say, very little.



Can I use any
12v relays, well shielded by copper boxes around them and what will

happen


to my 50ohms impedance?

Very little will happen to the 50 Ohm impedance if you use most
any common small relays that can handle 50 W of anything (with
enough fudge factors to withstand about 200 W of anything).

Use Ohm's Law of Resistance to find the RMS voltage and current
at 50 Ohms for 50 Watts. Multiply those by 1.414 to find the peaks.
Since the mobile antenna isn't a perfect 50 Ohms, multiply the
peaks by 4 as a safety margin. Those are the breakdown voltages
and current-carrying requirements of the relay contacts. Try to keep
the relay contact wiring to/from antenna and radio's coax as short as
possible.

It would be wise to use a "break-before-make" contact set on the
relays. That avoids having a "one-time-use" preamp circuit where
the transmit RF can wipe out the preamp input. Contact action
can be checked at DC and with a scope to be sure.

First of all, the typical (8 foot) whip isn't going to be at a common
impedance over the whole 10 m band. It's characteristics will
change between different vehicle shapes/conductive surfaces even
if at the same frequency in comparing vehicles. No matter, those
things work okay as they are...work better with some kind of
antenna matching network. Antenna pattern will vary depending
on the vehicle and its conductive surface shape and topography.
That is normal, nothing to be overly concerned about.

Only nit-pickers will get into hissy-fits on antenna impedance and
antenna patterns and so forth. :-)



As you can see I need all the guidance I can get:)

We can all help there although this thread might get into a different
direction with nit-picking that is sure to evolve...:-)

Relays are okay to use even if they have (what seems to be) "large"
contact arrangements (of around an inch or two). Some will insist
that such things will mess up the 50 Ohm impedance (of the coax)
but then they probably haven't measured actual mobile antenna
installations in detail, such as the connection between coax and
antenna base. No matter, any relay that can handle 200 W of
anything has contact structures that are about 0.01 wavelength
at 10 m and won't affect the characteristic impedance very much.

What I see as a potential problem is the relay contact making
the transmitter output directly into the preamp input, even for a
few milliseconds. That's blow-out time for the preamp with 50 W
potential output...50 W in a 50 Ohm system equals 50 V RMS
or 71 V peak, current is 1 A RMS or 1.4 A peak. [P = E^2 / R
and = I^2 x R] Relay contacts can operate in about 5mS (reed)
to 10mS (rotary spider-armature) to 100 mS (larger leaf-type
contact stacks). It may be that the transmitter output may not
reach full output in a short time (most probable) but that is an
unknown here.

A protective box for the relay is very good for environmental
protection (think salty water in northern climes in winter) but it
won't help otherwise for RF in this case. A totally-enclosing
ABS plastic box should be fine.


Putting on the skeptic's hat, I'd have to ask why the need of a
preamp? If more receive gain is desired, the safer place to do
that could be in the transceiver itself. The T/R switching is

already

there and presumably safe to use as it is.


retired (from regular hours) electronic engineer person



Hi,

Thanks for the prompt and very helpful response.

I do not know enough about radio circuits to fiddle with installing
additional preamps directly into the radio it' self.

I want to improve my reception as I normally travel with a group of

other

Land Rover owners who also have 29MH radios but they are not as fussy as

I

am about their installations and in many cases they can hear me but I

cannot

hear them.

I was thinking of using a double pole double throw relay.

I could fit a micro switch to the mic which activates before the press

to

talk switch activates the transmitter?

Thanks again
Greg




Even at 10 meters a modern radio should be fine, unless you have an
older rig or one that's got a reputation for being crappy I'd suspect a
fault with the radio or the antenna installation. I'd want to do a bit
of troubleshooting before I went off building modifications to something
that may just need repair.

At lower HF frequencies you can test your receiver noise level pretty
quickly by attaching an antenna -- if the atmospheric noise is
discernible then you have no trouble. I honestly don't know if this is
still a valid test at 10M.

If you can, see if you can swap rigs with somebody temporarily and go
driving. If the problem follows the rig then you have rig problems, if
it follows the car you have car problems. If you have no problem with
your car shut down but the noise level goes up with it running
(particularly if you hear the ignition) then you have ignition noise
problems. You may have an antenna that's more of a dummy load than a
radiator (and receiver).

If you still think you must have a preamp you should have a better time
installing the preamp inside the radio than outside -- there will be a
spot in the radio that comes after the T/R switch but before the mixer.
If the radio already has a preamp then you probably have one of the
aforementioned faults, and should be looking at "fix" rather than

"modify".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Hi Tim,

I don't think there's anything wrong with the radio, it is fairly new, I
was
hoping to snatch the last bit of a fading signal from the ether.

It is a very simple radio, the only controls are channel selection, squelch
and volume.

I know, get a more professional bit of kit, but I'm always challenged to
try
and improve things, which eventually costs more than buying the correct
thing in the first place.

I thing I've got the message, don't waste my time:((

Thanks to all for the help.

Greg














Nothing to add ... I just wanted to make this quote longer.

:^]

jw
K9RZZ


Well, you could have done that better by adding more than a one line
comment! I mean really. Now go away, think about what you did, and come
back and post at least 4 lines of sentence to add to the thread. Then
you'll be doing it right! 8^)

- Mike KB3EIA -


Yeah, you are correct.

I should have added more than a one liner.

Next time I'll know better.

Gee, I think I'll have pizza for dinner.

jw
K9RZZ

On 18 Jun 2004 04:36:49 GMT, (Avery Fineman)
wrote:

Putting on the skeptic's hat, I'd have to ask why the need of a
preamp? If more receive gain is desired, the safer place to do
that could be in the transceiver itself. The T/R switching is already
there and presumably safe to use as it is.

An exceedingly well-written explanation, if I may say so, Len.
You're of course right to suggest the above solution; the only problem
nowadays being the mind-boggling lack of space inside the new
transceivers!
As for the use of switching relays at RF; surely someone must make RF
relays specifically to preserve the 50 ohm characteristic impedance of
most transmission lines? After all, 29Mhz is one thing, but what is
one to do at say 2Ghz??
--

"What is now proved was once only imagin'd." - William Blake, 1793.

In article , Paul Burridge
writes:

On 18 Jun 2004 04:36:49 GMT, (Avery Fineman)
wrote:

Putting on the skeptic's hat, I'd have to ask why the need of a
preamp? If more receive gain is desired, the safer place to do
that could be in the transceiver itself. The T/R switching is already
there and presumably safe to use as it is.

An exceedingly well-written explanation, if I may say so, Len.
You're of course right to suggest the above solution; the only problem
nowadays being the mind-boggling lack of space inside the new
transceivers!
As for the use of switching relays at RF; surely someone must make RF
relays specifically to preserve the 50 ohm characteristic impedance of
most transmission lines? After all, 29Mhz is one thing, but what is
one to do at say 2Ghz??

Thank you, Paul.

There are many, many UHF-and-up relays available on the market,
but at a price most of us can't afford for hobby projects.

Those designs, as with UHF+ connectors, are arranged to minimize
the literal discontinuities that change spacing, dielectric material, and
so forth that alter the "characteristic impedance" of the equivalent
transmission line section of the structure. That's the price one pays.

There are some alternatives at lower powers. For an example, reed
relays can be fitted in brass sleeves (non-magnetic anything) and
form an acceptible coaxial transmission line section. Takes a bit of
experimentation to get the "outer conductor" v. inner conductor
ratios correct. I know those can operate to 3 GHz with less than
1.5 VSWR.

A high-environment relay (such as Military airborne) is the "rotary"
type. The magnetic core is attached to a plastic spider, all
arranged to move the spider in a short arc. Fingers on the spider
move low-length contacts welded to the hermetic seal pins of the
cylindrical case bottom. Moving contact length is roughly a
quarter inch, less than a cm. Very low contact-to-case-ground
capacitance, under 2 pFd, any combination. Allied Signal made
thousands of them and that style is still available, but for a
horrendous price, such as US$26 for a 6PDT, new. 2 A, 28 VDC
contact ratings...usually with a 28 VDC coil, but there are variants
of that for new designs. Hermetically sealed, those last forever.
I have some factory surplus left over from 1957, used in Hughes
Aircraft radar sets. Those can be found in various surplus gear
and most hobbyists just discard them, thinking they can't use
them for anything practical. I've had success using those in RF
switching up to 1 GHz with VSWRs less than 1.8:1.

A variation is the "crystal can" low-power relay, again sealed, but
contacts seldom go beyond 2PDT. Different contact-move
arrangements but the contact lead length is very short, minimum
inductance, and very low capacitance to case ground or other
contacts. Those have cases the size of an HC-6 quartz crystal
unit, some slightly larger but in the same dimension ratio.

Even the open-frame, leaf-spring contact arrangement can be used
at UHF with minimum discontinuity for balanced line switching.
Those require much examination as to the relative spacings of the
contact leaf springs, leaf widths, etc., and some are the wrong
dimensions and cannot be used. That kind of relay was never
designed for RF applications so there are no manufacturer's ratings
for that...one has to experiment.

For very quick checking of what can work and what can't, a TDR
or Time Domain Reflectometer can show that immediately. TDRs
have been around for three decades but are still horribly expensive.
Those who have them at work have an advantage of sneaking in
experimentation during lunch hour. :-)