Jim Lux wrote in
:

Owen Duffy wrote:
"998cc" wrote in
et:



The good thing is that your 5WL Yagi will have a good deal of
selectivity whereas it is certainly lacking in the '706IIG and in
many of the lower cost preamps (no money wasted on filtering).

I suspect the preamp makers would portray this as providing more
system engineering flexibility, letting you, the system builder,
decide how much filter selectivity (and corresponding loss/degradation
of NF) you want.

After all, if you were doing EME with a big array pointed up into the
sky, selectivity might not be such a big deal, but absolute lowest
system noise figure might be.

Hi Jim,

I suspect that was a bit tounge in cheek! I guess the manufacturers have
done us a bigger favour with hand helds in making them lighter by
minimising filtering.

If you want to fix an IC706IIG's front end selectivity here in Oz, one of
those Canadian filters for about $250 + $300 shipping is just the go.
Wait a minute, $550 to fix a $1000 radio's performance, and on just one
band... I don't think so.

I wrote an article on the performance of a modestly priced kit 144MHz
amplifier in the real world (as opposed to in a shielded room). I have
devised a test configuration that asseses what I term the 'realisable
sensitivity' which may be quite different to the specs derived in a
shielded room. The article is at http://www.vk1od.net/gt/144LNA/index.htm
..

The article also looks at the G/T improvements expected from other
configuration changes like a higher gain antenna and low loss coax, and
whether the preamp is located at the masthead or transceiver. Of course,
it all depends on one's own configuration, but the tools are there to
model a specific configuration.

Owen

Thank you Owen, Jim, Alan, Tam, Helmut, Jimmie and Jeff for all of the
great responses. I guess I should have provided more info
in the first post.

My station is in the foothills of Northern California at about 1200' elev
which is somewhat rural and is relatively quiet most of the time. Power
output is up to 500+ watts on VHF and the feedline to the antenna is about
90' of 1/2" Andrew Heliax hardline. The antenna is 43' up. Using my
previuos yagi, an M2 2M12, I worked stations as far as Oregon and San
Diego.

Good points on noise figures versus selctivity while considering noise floor
of the rig. I did try chercking the receiver noise with a Microlab dummy
load installed at the radio then with the antenna conneted. Yes, there is
more noise with the antenna connected. What is interesting is that with the
dummy load connected, the noise was just higher than wit the ope SO-239 at
the radio.

The responses have also steered my attention toward the 706G itself. While
it has been a great radio for several years, and offers fairly good
performance on many bands, designing this and similar multi-band radios
must include compromises. Perhaps I should be looking toward getting a rig
more tailored
to the 2m band and SSB mode? The Icom IC-910H comes to mind.

At this point, I will look more closely at the antenna/feedline system I
have and look into a better performing radio before investing in a
mast-mounted preamp that will introduce more connectors in the feedline and
which will likely require more maintenance as well.

Thanks again.
Russ W6OHM.

"998cc" wrote in
:

....
floor of the rig. I did try chercking the receiver noise with a
Microlab dummy load installed at the radio then with the antenna
conneted. Yes, there is more noise with the antenna connected. What

The higher the ratio of noise from the antenna to noise from the dummy
load, the less likely you are to get much benefit from a better receiver.

You need this ratio which tells you the ambient noise (albeit with IMD
noise rolled in) to predict how much a LNA or better receiver will
improved things.

Of course, by 'improve' I mean better S/N ratio, not simply higher S
meter deflection.

Alternatively, you suck it and see.

is interesting is that with the dummy load connected, the noise was
just higher than wit the ope SO-239 at the radio.

I assume that was to say "with the open SO-239".

I don't really know what you can conclude, because you don't know enough
about the radio when the input jack is o/c.

Owen

"998cc" wrote in message
et...
I am looking for ways to improve my receive for 2-meter SSB weak signal
work. I am running an Icom 706 MkIIG and a M2 5WL yagi. I understand mast
mounted preamps bring the signal up significantly.
Doe any one here use one for 2M SSB? What are your experiences?

Thanks
Russ

Stack another yagi so you can reply to what you hear.

"Hal Rosser" wrote in message
...

"998cc" wrote in message
et...
I am looking for ways to improve my receive for 2-meter SSB weak signal
work. I am running an Icom 706 MkIIG and a M2 5WL yagi. I understand
mast mounted preamps bring the signal up significantly.
Doe any one here use one for 2M SSB? What are your experiences?

Thanks
Russ

Stack another yagi so you can reply to what you hear.


If he has room for it, this may very well be the best way to go. Gets away
from finding an amp that can take 500W on transmit, and does away with
complicated sequencing, which, if not done right could blow the amp if you
accidentally use VOX. For starters, replacing the 706 will probably result
in the best fun factor.

Tam/WB2TT

Stack another yagi so you can reply to what you hear.


If he has room for it, this may very well be the best way to go. Gets away
from finding an amp that can take 500W on transmit, and does away with
complicated sequencing, which, if not done right could blow the amp if
you accidentally use VOX. For starters, replacing the 706 will probably
result in the best fun factor.

Tam/WB2TT

Tam, that would be great, but I live in a community with CC&R's and antenna
restrictions. I was barely able to get permission for my single yagi by
using 911 and the Loma Prieta earthquake as bases for arguments. I really
don't want to push the issue too hard. I do agree that looking into a
better radio is probably the first step. If they made a VHF/UHF radio with
similar features to the Icom 756 Pro-III, I'd grab it in a heartbeat.
Otherwise, the antenna and feedline will be optimized as time permits.

73.

Russ
W6OHM

"998cc" wrote in message
...

Stack another yagi so you can reply to what you hear.


If he has room for it, this may very well be the best way to go. Gets
away from finding an amp that can take 500W on transmit, and does away
with complicated sequencing, which, if not done right could blow the amp
if you accidentally use VOX. For starters, replacing the 706 will
probably result in the best fun factor.

Tam/WB2TT

Tam, that would be great, but I live in a community with CC&R's and
antenna restrictions. I was barely able to get permission for my single
yagi by using 911 and the Loma Prieta earthquake as bases for arguments.
I really don't want to push the issue too hard. I do agree that looking
into a better radio is probably the first step. If they made a VHF/UHF
radio with similar features to the Icom 756 Pro-III, I'd grab it in a
heartbeat. Otherwise, the antenna and feedline will be optimized as time
permits.

73.

Russ
W6OHM

A few years ago I bought an FT847 as a backup/VHF/UHF rig. Wish I had spent
a few hundred more for a TS2000. A friend has one, and it is very nice if
you don't mind the art deco styling.

Tam/WB2TT

"Tam/WB2TT" wrote in
:

A few years ago I bought an FT847 as a backup/VHF/UHF rig. Wish I had
spent a few hundred more for a TS2000. A friend has one, and it is
very nice if you don't mind the art deco styling.

Talking of 144Mhz (the context of the original posting)...

Competition means that all these guys claim similar SSB telephony
sensitivity (measured in a shielded room) which corresponds to an
equivalent noise temperature of about 600K.

A key issue is that on a real antenna, this 600K could easily be 6000K
due to noise created by IMD in the receiver, and that doesn't show up on
the spec sheets, you can only assess that in situ, and it varies from
receiver to receiver and antenna to antenna. (The same issue exists with
any preamp or LNA.)

Carrying on with the (slightly unrealistic) best case of a 600K receiver,
used without a preamp and with say 1dB of line loss, your receiver
equivalent noise temperature is nudging 700K which is probably adequate
unless you are in the quietest of locations.

What 'works' for someone else isn't guaranteed to 'work' the same for
everyone.

An interesting illustration was the case of one of my VHF neighbours who
had a narrow filter (of the type used in two way radio duplexors)
installed between his preamp and transceiver. I asked why that
configuration and the answer was to minimise the loss in front of the
preamp (0.5dB of filter loss would add 35K to the system noise
temperature). On my suggestion, he moved it in front of the preamp and
compared the difference in S/N on a beacon and it was noticeably better
in front of the preamp, so despite the additional 35K, it had reduced
preamp IMD noise by more than that amount.

You don't make contacts in a shielded room, don't depend entirely on
shielded room perspectives for station design.

Owen