A supid question from someone who thought he knew a bit about
electronics...

I’ve bought a mini hi-fi to place permanently in the kitchen but the
FM reception is bad and I want to treat it to an aerial in the loft.
The issues a

- The FM radio only has a short single wire for an aerial, no coax
socket.
- The transmission cable will have to run up a duct that’s full of
mains wiring
- It also runs close to the computer upstairs, which emits
interference

My intention was to run coax cable up the duct, hitched to a
four-element (or thereabouts) aerial in the loft. But now I’m totally
confused by ribbons, coax, baluns, impedence (not stated by the
manufacturer) and all the other trappings of this science.

Questions:

Trawling the net, some pundits suggest twin flat ribbon as a simple
solution. But surely the stuff is unscreened and will be highly
susceptible to stray radiation, reflections etc if I run it up the
duct? Am I right in thinking that, with coax cable, the ‘outer’ acts
as a shield to the signal running through the ‘inner’?

Every configuration, whether it’s ribbon or coax, ends up with two
conductors. Where does the second one go in the absence of a proper
socket? To earth / chassis / nowhere? (There's also a socket for an AM
coiled aerial, if that helps).

Signal strength in our area is fairly weak but does improve with
height. All I want is to boost the signal on main national (UK)
stations to an acceptable level for stereo.

At the moment I have a length of Twin & Earth lighting cable running
up the duct - we used it as a dummy just to get something fed through
(nightmare job!). Don’t suppose this will be any use? I was half
hoping that this cable, running about 15ft vertically, might in itself
provide enough extra signal, but attaching it to the radio’s aerial
improves things only a little.. We could always use it to pull through
a proprietory cable.

Help please. I don’t know where to start - especially with the single
wire issue. I was hoping to rig up something functional without a lot
of coils and calculations!

Thanks
Paul

Hi Paul

Yes it looks like you have a real problem here..

- Yes running ribbon up a power duct is not a good idea. Generally
speaking ribbon is pretty good in rejecting noise when there is it least
a few inches of air spacing between it and metal objects. The nature of
ribbon (or balanced feeder) is that any noise induced in one wire will
also appear in the other and they will add out of phase in the receiver.

- Yes coax is a better idea, but be aware that the length of coax will
also receive in its own right if both the radio and antenna ends arent
terminated in its surge impedence (ie 75 ohms). There are some
variations on this statement but I wont go deeper.

- The wire coming out the back of the radio is probably loosely
referenced against its metal chassis. Unless you want to go inside the
radio your only real option is to join the coax outer to the chassis
near where the single wire exits, and the centre conductor to the wire
itself (after cutting it short or removing/replacing it)

- The above of course gives you an unknown RX load impedence.

- The existing power twin and earth might be usable except for the
possibility of induced noise. I assume you mounted a dipole at the top
of it? I would guess that it would be maybe 100-200 ohms impedence but I
dont know the effect of the earth wire to that. If you had nothing
better to do you could always make it into a tuned feeder length or
multiples of 1/2 wavelengths. Dunno if its worth the effort though.
(Tuned feeder installations allow you to have any feeder impedence
provided the source and load are the same impedence. Doubt its any point
doing in an RX system but worth mentioning out of interest)

- If patience was in huge supply I'd try matching at the receiver input
for a 75 ohm cable. I'd go inside the radio to do this.

- If the signal is still way too low it might be worthwhile
buying/building a masthead preamp to go near/on the antenna. Then noise
induced by the power duct and computer would then be lower. You may also
want to notch other strong stations at the input of the preamp with a
series of 1/4 wave stubs so the preamp doesnt overload.

Hope this helps - or at least doesnt confuse!

Cheers Bob (Sydney Australia)



Paul Davies wrote:
A supid question from someone who thought he knew a bit about
electronics...

Wow Bob, that was some reply. Very much appreciated.

-... but be aware that the length of coax will
also receive in its own right if both the radio and antenna ends arent
terminated in its surge impedence (ie 75 ohms).

Couldn't quite understand that (word ot two missing?)


I assume you mounted a dipole at the top
of it?

Nope, the top end is just sitting on the floor of the loft, the
surplus coiled up. Last night I did get a reasonably clean signal from
Radio 3 in stereo so we're not looking at a need for massive gains.

- If patience was in huge supply I'd try matching at the receiver input
for a 75 ohm cable. I'd go inside the radio to do this.

Would this need a transformer, inductance/capacitance circuit, static
resistor, or what??

Bob, could you explain one mystery to me? A normal aerial arrangement,
whether fed by ribbon or coax, is effectively one continuous loop of
wire. DC resistance is at most a few ohms - ie pretty much a dead
short. Impedence is resistance to AC, ie the signal, but the fact that
impedence is present doesn't remove the dead short. So how come
anything gets around the loop at all?!!

Or is it that the entire loop can be seen as one side of a circuit for
which it provides a potential difference?

And another: Where does this PD occur? Across the receiving element of
the aerial, or within the element relative to earth? Does this mean an
aerial 'loop" works differently from a single long wire?

You probably see what I'm getting at.

Thanks again
Paul


On Tue, 15 Mar 2005 05:36:34 +1100, Bob Bob
wrote:

Hi Paul

Yes it looks like you have a real problem here..

Hi Paul

Couldn't quite understand that (word ot two missing?)

See if I can explain better..

A coax line will radiate (if its a transmitting circuit) and receive
signal (as a receiving cct) if it isnt terminated into the (same)
impedence at both ends of the cable that is the same as the cable
itself. In other words the shield wire isnt always an effective screen.
What is also an issue is if the symmetry of the source and load isnt
preserved. ie if you feed a dipole direct with coax the "balanced"
aerial and "unbalanced" coax will add up to an amount of line radiation.
A balun is often used to balance an unbalanced feeder. This can either
be a wire wound transformer or loops of coax. Feeding a vertical 1/4
wave whip is a good example of feeding an unbalanced antenna with
unbalanced cable..

You can probably imagine that this coax radiating/receiving issue is a
big problem when the cable runs through a cars engine compartment.

Would this need a transformer, inductance/capacitance circuit, static
resistor, or what??

Well that depends on the cct inside. Being a wide band device the
impedence will vary greatly and no doubt have a reactive component. I
have no idea what the norm is for FM broadcast receivers, even those
with 75 ohm or 300ohm connectors! I'd suspect a simple L/C matching
circuit would do it but might even opt for a small toroidal balun. I'd
just assume that the Z would be about 300 ohms and create a 4:1 RF
transformer (thats 2:1 turns) for the 75 ohm coax. The balun will tend
to resolve any reactance and balance problem as well.

My personal preference for an antenna would be a single quad loop fed
over the last 1/4 wave with a piece of RG62 93 ohm coax. This will match
the Z of the antenna (about 110 ohms) with the coax. I'd also coil up
about 5 turns of coax into a 3" loop where it feeds the antenna. This
will resolve the balanced/unbalanced line radiation problem. This is of
course more a transmitter config and given that you want to use over a
wide range of freqs the 1/4 matching section would be a mute point.

Bob, could you explain one mystery to me? A normal aerial arrangement,
whether fed by ribbon or coax, is effectively one continuous loop of
wire. DC resistance is at most a few ohms - ie pretty much a dead
short. Impedence is resistance to AC, ie the signal, but the fact that
impedence is present doesn't remove the dead short. So how come
anything gets around the loop at all?!!

You have to think of the antenna as a tuned AC circuit. In fact
completely throw away any thoughts of a DC circuit havning any effect
whatsoever. Same kind of logic as a power transformer not looking like a
short circuit. Think of the antenna as being feed by an instantaneous
voltage that takes a fixed time to get from the feedpoint to where there
is a "short cicruit" in the wire. The signal from one side of the coax
arrives "in phase" with the one from the other side so no current flows
between them. The trick is in the length of the wire in question. For a
folded dipole the impedence looks like about 300 ohms provided the
antenna is tuned to resonance. If you depart from resonance the
conjugate impedence (resistive plus reactive) will always rise. If you
start feeding this antenna at twice the freqency for what it was
designed you end up with a very high impedence (say 10K plus ohms). You
are now "voltage feeding" the thing.

It can and is a little more compex than this and I am not sure I have
done it justice. Try another analogy. If you are pushing a child on a
swing in sync with it, the energy you put in will be effectively used.
If however you push in the exact opposite sense (by timing 180 degrees
out of phase) you hit what look like a short circuit. Think in terms of
instantaneous voltage in different parts of the antenna that change over
time.

And another: Where does this PD occur? Across the receiving element of
the aerial, or within the element relative to earth? Does this mean an
aerial 'loop" works differently from a single long wire?

Really testing my basic theory here! grin

For most applications you would tend to think that the volts would occur
at the antenna feedpoint as an AC value 180 degrees difference from one
side to the other. You can draw the instantaneous current and voltage on
the wire if you like (and this is handy for working out the shape of the
radiated pattern) but probably isnt helpful in your case.

Also think in terms of maximum power transfer with soruce and load Z...
Reactance of course complicates that and the *real* theory involves
looking at capacitive and indictive reactance..

I hope this all means something to you!

Cheers Bob

Bob Bob wrote:
. . .

A coax line will radiate (if its a transmitting circuit) and receive
signal (as a receiving cct) if it isnt terminated into the (same)
impedence at both ends of the cable that is the same as the cable
itself. In other words the shield wire isnt always an effective screen.

This is a common misconception, but it's not true -- the presence of
current on the outside of a coax line (which is the cause of radiation
and signal pickup) has nothing to do with the impedance match at either
end of the line.

What is also an issue is if the symmetry of the source and load isnt
preserved. ie if you feed a dipole direct with coax the "balanced"
aerial and "unbalanced" coax will add up to an amount of line radiation.

That's not entirely correct, either. A coax line can be just as
"balanced" as twinlead, and twinlead can be just as "unbalanced" as
coax. Imbalance is caused by other factors besides physical symmetry --
see "Baluns: What They Do and How They Do It" in the ARRL Antenna
Compendium, Vol. 2, for example.

. . .


You have to think of the antenna as a tuned AC circuit. In fact
completely throw away any thoughts of a DC circuit havning any effect
whatsoever. Same kind of logic as a power transformer not looking like a
short circuit. Think of the antenna as being feed by an instantaneous
voltage that takes a fixed time to get from the feedpoint to where there
is a "short cicruit" in the wire. The signal from one side of the coax
arrives "in phase" with the one from the other side so no current flows
between them. The trick is in the length of the wire in question. . .

I don't quite follow this, so I might be misinterpreting what was said,
but it doesn't sound quite right. A small loop has a small value of
resistance but a moderate amount of inductance. So the impedance
(composed of the resistance and inductive reactance), although low, is
considerably higher than just the low-frequency value. Although the
reactance provides the lion's share of the impedance, even the
resistance is quite a bit higher than the low frequency value due to
skin effect. As the antenna gets larger, the inductance increases and,
due to capacitive effects, the antenna eventually becomes resonant.
That's the frequency at which the inductive and capacitive reactances
just cancel. A short dipole antenna, on the other hand, has a much lower
impedance at RF than its low frequency value, primarily due to the
capacitance between the two conductors. Its impedance is composed of a
small resistance and a large capacitive reactance.

The current on the center conductor of the coax isn't in phase with the
current on the shield -- it's exactly out of phase with the current on
the inside of the shield. So the same amount of current that flows out
of the inner conductor flows back along the inside of the shield. In
this way, an antenna behaves just like any other electric circuit.

. . .

Roy Lewallen, W7EL

Yes Bob, what you're saying does make some sense and will make even
more after I've read it a few times. So does Roy's.

I've got a pretty good handle on the basics, having a physics degree
and a long interest in hobby electronics. But as black arts go, the
workings of antennae and tuned circuits come pretty high on the list!

Some experimentation is obviously needed. I'll start with a simple
dipole and a run of coax, the outer screen attached to chassis. How
does that sound? Is it worth buying a proper aerial or, being in the
loft, would a home-made rig suffice? What would you suggest
design-wise?

Money isn't really an issue - just that there's no point in buying kit
that turns out to be unnecessary. As I said, the signal is already
improved by hooking up the flying aerial lead to the wire in the duct.

Meanwhile I'll let you guys fight out the theory!

Many thanks for your advice

Paul

Paul Davies has bad FM reception in his kitchen and would like to try an
external antenna. Problem is that transmission lines require 2 wires or
else they become part of the antenna themselves. The FM set has only a
lone antenna wire and no ground connection.

Note that many FM sets have no antenna wire but use a clamp on the power
cord to capacitively couple signal from the powerline.

Since Paul has an antenna wire, he can use the same sort of capacitive
connection but to connect with the chassis (ground) that many other sets
use for the "signal" connection to the set.

The clamp Paul adds around his power cord near the set chassis should
connect to the inside of his coax cable shield, directly or indirectly.
The antenna wire of his set can be shortened to insignificance and
connected to the inner conductor of his coax.

In the loft, Paul can make a ground plane antenna to connect with his
coax to the radio. The wavelength is 3 meters or about 10 feet. He could
spread1/4-wave radials in a balanced and symmetrical manner to construct
the horizontal part of his antenna. This would screen the vertical part
of his antenna from interference and provide a terminal for the ground
plane`s circulating capacitive current. Paul`s coax shield would connect
with all the radial wires (2, 4, 6, or 8). Odd numbers work too so long
as symmetry is preserved.

The vertical part of his antenna is a 2.5 ft. length of wire above the
center of the ground plane. The center conductor of his coax connects to
the vertical wire.

Paul could wind a half dozen turns in his coax directly beneath his
ground plane to discourage signal on the outside of the coax. A W2DU
ferrite choke could also be used. Chokes at both ends of the coax might
be better. But operation may be entirely satisfactory with no chokes at
all. It depends on local conditions.

Paul would have connections at both ends of his coax so it could operate
as a proper transmission line,

Best regards, Richard Harrison, KB5WZI

Egads, you blokes are helpful.

So let me get this right. I am trying to picture Richard's 1/4 wave
radial arrangement. I visualise an upturned umbrella. The spokes are
each 2.5ft long, joined at the middle to the coax shield. The
umbella's handle is also 2.5ft long, connected to the coax inner.

(Does it matter that the local transmitters, so I gather, use
horizontal polarization?)

At the other end the inner is conected to the stub of aerial wire, the
shield to a clamp around the mains cable.

Does the umbrella analogy work for you?

Questions:
How big would the clamp be?
Why not connect shield to chassis?

Not quite clear on two other things, Richard
the inside of his coax cable shield,
Contradiction in terms?

but to connect with the chassis (ground) that many other sets
use for the "signal" connection to the set.
Why should anyone want to connect the signal to ground?

Thanks again
Paul

Paul wrote:
"Does it matter that the local transmitters, so I gather, use horizontal
polarization?"

Good question. Since FM reception became popular in vehicles,
broadcasters have had to add vertical transmission to their broadcasts
to better serve the mobiles.

Best regards, Richard Harrison, KB5WZI

Hi Roy

Mmmmm, yet another base belief falls screaming from the sky! grin

It would be interesting to know how it happened (the misconception that
is). The low level thinking (for me) on a transmission line is that for
a pair of wires, the signal on one side is always 180 degrees out of
phase with the other. Any noise induced by a magnetic field would be in
the same phase hence common mode rejection means it isnt "seen" by a RX.
I cant help thinking that unbalanced line is somewhat asymetrical in
that the diameter of the outer conductor would *somehow* have a non
trivial width when it comes to the wavelength of the frequency in use.
Needless to say I havent gone to the extent of plotting magnetic lines
and rereading my base theory stuff. Some things one just has to accept!

I get the *feeling* that balanced feeder is less likely to radiate than
unbalanced.

Why does one use triax in some situations? (cable damage and inadvertent
DC supply grounding aside)

One hopes it is a fair statement to say that any inbalance in the
current on either side of the transmission line (or phase shift ne 180
deg) will result in line radiation.. (However one gets it)

I have grabbed the ARRL balun PDF from the Eznec site and will see how I
go... Have to pay some bills first!

I was trying to explain why an antenna (folded dipole or qaud) isnt
really a short circuit at the operating frequency, without referring to
terms like Xc, Xl, and resonance. I see I failed! What I was trying to
portray was that for the amount of time and distance (aka wavelength)
that the electrons had to travel around the antenna any instant in time
and at any specifc place would never see a short circuit. Or of you like
AC is very different to DC.

What I really want to know is whether Paul's FM RX is now working!

Cheers Bob


Roy Lewallen wrote:
-----snips----

You have to think of the antenna as a tuned AC circuit. In fact
completely throw away any thoughts of a DC circuit havning any effect
whatsoever. Same kind of logic as a power transformer not looking like
a short circuit. Think of the antenna as being feed by an
instantaneous voltage that takes a fixed time to get from the
feedpoint to where there is a "short cicruit" in the wire. The signal
from one side of the coax arrives "in phase" with the one from the
other side so no current flows between them. The trick is in the
length of the wire in question. . .


I don't quite follow this, so I might be misinterpreting what was said,
but it doesn't sound quite right.