[OT] Rcvr antenna input impedance
 

Hi,

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably [1]. I think this means the receiver's input
impedance is much greater than the normal 50/75 ohms. This makes
me wonder two things:

- How can I measure the approximate value of the receivers input
Z? I don't want to know the exact value, just to know if it is in
the 500 / 1K / 10K range. I guess I can do this by paralleling
increasingly valued resistors until there's not much effect on
signal and assume that it about half that resistor's value.

- Having a 50 ohm feedline, would I benefit by using a 4:1 balun
in reverse at the rcvr input? What kind of balun could I use for
that? It would be great if it also provided some improvement of
CMRR for noise picked by the feedline itself.

[1] I use a 100nF capacitor to isolate the 4~5 V DC level at the
antenna input. I guess this DC may be there to feed some kind of
active antenna.

--
Toni

"Auto" = prefijo griego que significa "no funciona"

On Wed, 15 Sep 2004 11:08:21 +0200, Toni
wrote:

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably

Hi Toni,

This is something you could test yourself:
- Having a 50 ohm feedline, would I benefit by using a 4:1 balun
in reverse at the rcvr input?
with a before/after 300 ohm resistor application. Sure it isn't a 330
Ohm resistor? 300 is not a standard value. Also, is there any chance
this resistor is wire wound? Makes an important difference!

Alternatively, the same phenomenon should occur if you put the
resistor in series. Be careful to maintain ground in all tests.

Finally, what are you using as an input reference? The signal from
this same 50 Ohm feedline?

73's
Richard Clark, KB7QHC

On Wed, 15 Sep 2004 11:08:21 +0200, Toni
wrote:

Hi,

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably [1]. I think this means the receiver's input
impedance is much greater than the normal 50/75 ohms. This makes
me wonder two things:

- How can I measure the approximate value of the receivers input
Z? I don't want to know the exact value, just to know if it is in
the 500 / 1K / 10K range. I guess I can do this by paralleling
increasingly valued resistors until there's not much effect on
signal and assume that it about half that resistor's value.

- Having a 50 ohm feedline, would I benefit by using a 4:1 balun
in reverse at the rcvr input? What kind of balun could I use for
that? It would be great if it also provided some improvement of
CMRR for noise picked by the feedline itself.

[1] I use a 100nF capacitor to isolate the 4~5 V DC level at the
antenna input. I guess this DC may be there to feed some kind of
active antenna.

Buy a cheap passive antenna tuner, e.g. the YAESU FRT-7700
or the one from MFJ Enterprises.

Forget that idea with the resistors.

w.

Toni wrote:
Hi,

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably [1]. I think this means the receiver's input
impedance is much greater than the normal 50/75 ohms. This makes
me wonder two things:


The input impedance will vary depending on the receive frequency.
It's something that isn't much worried about in radio design for
SW. As the radio's AVC circuits will make up for gain losses
due to mismatches. Also realize that the receiver's front end
noise is much less than the amount of noise produced by the
natural and man made environment. That would limit how much DX
you can do. A perfect match of antenna to front end would just
deliver more noise along with signal, and would be indistinguishable
from a mismatch with the AVC making up the missing gain. I'm
talking about around 10dB mismatches here.

Hi Richard,

En Richard Clark va escriure en Wed, 15 Sep 2004 15:43:55 GMT:

On Wed, 15 Sep 2004 11:08:21 +0200, Toni
wrote:

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably

This is something you could test yourself:
- Having a 50 ohm feedline, would I benefit by using a 4:1 balun
in reverse at the rcvr input?
with a before/after 300 ohm resistor application. Sure it isn't a 330
Ohm resistor? 300 is not a standard value. Also, is there any chance
this resistor is wire wound? Makes an important difference!

It probably is 330 ohm, it wrote 300 ohms just to show in what
order of value it was. It is not a wire wound, probably a bad
quality carbon resistor, but even in the firs case I think it
would only affect if used in series, not if used in paralel
(where the extra inductance would simply put a higher value Z in
parallel to the source)

Alternatively, the same phenomenon should occur if you put the
resistor in series. Be careful to maintain ground in all tests.

Yes, the series circuit would also probably work foro estimating
the input Z, but with the parallel it is easier to solder one leg
of the resistor to ground and touch/don't touch the signal wire
to compare both situations.

Finally, what are you using as an input reference? The signal from
this same 50 Ohm feedline?

Yes, I just turn to SSB where AGC seems to have much less effect
(if working at all) and "touch/don't touch" the resitor. The
effect on volume is very noticeable. Unfortunately the ICF-7600G
has no s-meter.

73's
Richard Clark, KB7QHC

Thanks,

--
Toni

"Auto" = prefijo griego que significa "no funciona"

Hi Helmut,

En Helmut Wabnig va escriure en Wed, 15 Sep 2004 17:47:43 +0200:

On Wed, 15 Sep 2004 11:08:21 +0200, Toni
wrote:

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably [1]. I think this means the receiver's input
impedance is much greater than the normal 50/75 ohms. This makes
me wonder two things:

- How can I measure the approximate value of the receivers input
Z? I don't want to know the exact value, just to know if it is in
the 500 / 1K / 10K range. I guess I can do this by paralleling
increasingly valued resistors until there's not much effect on
signal and assume that it about half that resistor's value.

- Having a 50 ohm feedline, would I benefit by using a 4:1 balun
in reverse at the rcvr input? What kind of balun could I use for
that? It would be great if it also provided some improvement of
CMRR for noise picked by the feedline itself.

Buy a cheap passive antenna tuner, e.g. the YAESU FRT-7700
or the one from MFJ Enterprises.

This is just what I wanted to avoid. I just wanted a
better-than-nothing approach that doesn't require retuning at
every band change. Aside, having no s-meter it would probably be
difficult to calibrate the tuner.

Something different would be a pasive preselector. I will
probably buy or make one, but this is a different beast.

Forget that idea with the resistors.

You will excuse me but I don't see why. It may not be a high-tech
computer-controled impedance analyzer but I just need
order-of-magnitude precision, I don't even need the first digit
to be much accurate.

Thanks for your comments

--
Toni

"Auto" = prefijo griego que significa "no funciona"

En Robert Casey va escriure en Wed, 15 Sep 2004 18:59:23 GMT:

Toni wrote:

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably [1]. I think this means the receiver's input
impedance is much greater than the normal 50/75 ohms. This makes
me wonder two things:

The input impedance will vary depending on the receive frequency.
It's something that isn't much worried about in radio design for
SW.

Yes, I think this must be the case, but even if it varies I think
it will go up/down to double/half or triple/one third, I don't
think (hope not) it goes to 10x / 1/10th. That's why I don't want
to try to perfect-match, just same order of magnitude match

As the radio's AVC circuits will make up for gain losses due
to mismatches. Also realize that the receiver's front end
noise is much less than the amount of noise produced by the
natural and man made environment. That would limit how much DX
you can do. A perfect match of antenna to front end would just
deliver more noise along with signal, and would be indistinguishable
from a mismatch with the AVC making up the missing gain. I'm
talking about around 10dB mismatches here.

Well, the case is that I've been playing, among others, with
untuned shielded loops, and they really help reduce interference
by carefully turning them. Unfortunately they also produce very
low signal level, close to the point of not hearing much
difference in background noise when connecting/disconnecting the
antenna. That't why I'd like to gain those extra 5~10 dB by
mathing the receiver to the feedline.

Aside, the ICF-7600G is possibly the best receiver I have had so
far in sensitivity and selectivity, matching, if not improving,
my TS-570-DG transceiver. It is also great for carryng arround.
(I know I shouldn't make free publicity...)

Thanks for your comments

--
Toni

"Auto" = prefijo griego que significa "no funciona"

Toni wrote:
"---if I parallel a 300 ohm resistor to the antenna input the received
signal drops considerably."

Toni says he is not interested in high accuracy, just an idea of what
his receiver impeance is. He is on the right track. If two identical
resistors are paralleled across a voltage source, each takes the same
current. The portable radio antenna is probably a current source due to
its high capacitive reactance in the too-short antenna. That makes the
voltage out proportional to the impedance loading the antenna.

Forget the .7 vs. .5 of the power vs. volts relations. All you want is a
rough estimate of the receiver`s input impedance. Just use common carbon
resistors to find which cuts your received level from a weak signal (not
to get AVC involved) to about half what it is without the parallel
resistor.

Best regards, Richard Harrison, KB5WZI

Toni wrote in message

Well, the case is that I've been playing, among others, with
untuned shielded loops, and they really help reduce interference
by carefully turning them. Unfortunately they also produce very
low signal level, close to the point of not hearing much
difference in background noise when connecting/disconnecting the
antenna. That't why I'd like to gain those extra 5~10 dB by
mathing the receiver to the feedline.

Why don't you just tune the loop? That would cure the level problem,
and also reduce out of band signals. I do agree with the other poster
though. It doesn't take much to provide a usable s/n ratio on HF. If
the noise level increases when you connect the antenna, it should be a
good enough match. MK

On Wed, 15 Sep 2004 11:08:21 +0200, Toni
wrote:

Hi,

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably [1]. I think this means the receiver's input
impedance is much greater than the normal 50/75 ohms. This makes
me wonder two things:

- How can I measure the approximate value of the receivers input
Z?

Normally done with a noise bridge.

Am scratching my head what to do, if there is no noise bridge....

w.

On Fri, 17 Sep 2004 13:20:22 +0200, ************** wrote:

Normally done with a noise bridge.

Am scratching my head what to do, if there is no noise bridge....

Build one: 4 resistors, Zener diode, battery.

En Richard Clark va escriure en Fri, 17 Sep 2004 18:41:36 GMT:

On Fri, 17 Sep 2004 13:20:22 +0200, ************** wrote:

Am scratching my head what to do, if there is no noise bridge....

Hi Richard,

Build one: 4 resistors, Zener diode, battery.

That is the easy part, the not so easy is that AFAIK you need a
(mediumly good) transformer to get the signal from the noise
bridge to a receiver without altering the bridge.

I had thought about building a simple bridge, but ferrite cores
are close to impossible to get in Barcelona (Spain). You get them
but with no info at all, just size and color. If you ask anything
else the shop person will look at you as a trouble-maker.

Appart from that, I have finally found the schematics of my
receiver. Some low-pas and diode limiter and strait to the gate
of a fet. This explains the "not exactly 50 ohms" thing.

73's

--
Toni

"Auto" = prefijo griego que significa "no funciona"

Toni wrote:
"This explains the "not exactly 50 ohms" thing."

The FET input resistance is high and a shunt diode probably has high
impedance at low signals.

The antenna appears as a generator at its resonant frequency with an
internal resistance equal to its radiation resistance. As frequency
lowers, so does radiation resistance. The antenna`s series capacitive
reactance rises rapidly too. Due to lower antenna loading, a high
receiver input impedance does better with a too-short antenna than does
a 50-ohm input. But high-impedance input may be more susceptible to
interference and overload.

Maximum power transfer to a receiver occurs when there is a conjugate
match between the antenna and its receiver. This may, however, not give
the best noise performance.

Best regards, Richard Harrison, KB5WZI

On Mon, 20 Sep 2004 08:56:22 +0200, Toni
wrote:
Build one: 4 resistors, Zener diode, battery.

That is the easy part, the not so easy is that AFAIK you need a
(mediumly good) transformer to get the signal from the noise
bridge to a receiver without altering the bridge.

Hi Toni,

Not necessary, but not impossible either. We will proceed as if you
do need one.

I had thought about building a simple bridge, but ferrite cores
are close to impossible to get in Barcelona (Spain). You get them
but with no info at all, just size and color.

Then those have a high chance of being Powdered Iron core. Wrong
cores.

Find a swap meet where old computers are being offered, especially old
cables. Look for those power or video connectors that have a plastic
cylinder at one end, 1 or 2 cm near the actual connector. Cut away
the plastic, cut away the wires: you have a high chance that you are
holding a ferrite core that will work.

Appart from that, I have finally found the schematics of my
receiver. Some low-pas and diode limiter and strait to the gate
of a fet. This explains the "not exactly 50 ohms" thing.

But it doesn't tell you what Z it IS.

73's
Richard Clark, KB7QHC

Hi richard.


En Richard Clark va escriure en Mon, 20 Sep 2004 16:20:45 GMT:

On Mon, 20 Sep 2004 08:56:22 +0200, Toni
wrote:
Build one: 4 resistors, Zener diode, battery.

That is the easy part, the not so easy is that AFAIK you need a
(mediumly good) transformer to get the signal from the noise
bridge to a receiver without altering the bridge.

Hi Toni,

Not necessary, but not impossible either. We will proceed as if you
do need one.

I had thought about building a simple bridge, but ferrite cores
are close to impossible to get in Barcelona (Spain). You get them
but with no info at all, just size and color.

Then those have a high chance of being Powdered Iron core. Wrong
cores.

Find a swap meet where old computers are being offered, especially old
cables. Look for those power or video connectors that have a plastic
cylinder at one end, 1 or 2 cm near the actual connector. Cut away
the plastic, cut away the wires: you have a high chance that you are
holding a ferrite core that will work.

Now, this is a good idea, I'll try it. I know you can get those
cores from online shops, I just don't like buying over the
internet.

Appart from that, I have finally found the schematics of my
receiver. Some low-pas and diode limiter and strait to the gate
of a fet. This explains the "not exactly 50 ohms" thing.

But it doesn't tell you what Z it IS.

No, but now I know it is probably quite high, so I can try the
reversed 4:1 (or more) balun idea.

73s

--
Toni

"Auto" = prefijo griego que significa "no funciona"

Hi Richard,

En Richard Harrison va escriure en Mon, 20 Sep 2004 10:24:54
-0500:

Toni wrote:
"This explains the "not exactly 50 ohms" thing."

The FET input resistance is high and a shunt diode probably has high
impedance at low signals.

Due to lower antenna loading, a high
receiver input impedance does better with a too-short antenna than does
a 50-ohm input. But high-impedance input may be more susceptible to
interference and overload.

Maximum power transfer to a receiver occurs when there is a conjugate
match between the antenna and its receiver. This may, however, not give
the best noise performance.

Yes, but my main objective for all this is feeding the receiver
with the signal comming from the antenna well above the roof, and
(try to) avoid feeding the noise picked by the feedline on it's
way down. Light dimmers, fluorescent tubes, the computer, TV,
etc. I can notice a noise difference when turning them on/off,
and that's what I want to solve/improve.

73s,

--
Toni

"Auto" = prefijo griego que significa "no funciona"

Toni wrote:
"Yes, but my main objective for all this is feeding the receiver with a
signal coming from the antenna well above te roof and (try to) avoid
feeding the noise picked by the feedline on its way down."

OK. Coax is almost impenetrable at HF. But a directly connected antenna
can accept signal from the coax ecterior unless a balun is used at the
antenna end of the coax.

I recall using a General Electric "V-Doublet" receiving antenna kit in
1938 that used twisted pair for the transmission line. Broadband
transformers were used at each end of the transmission line. They were
interchangeable. The high impedance of the antenna and receiver were
matched to about 100-ohms, the approximate impedance of the twisted
pair.

It worked well on all bands for the multiband GE receiver. The
"V-Doublet" antenna system ignored interference to its transmission
line.

Coax could be used to replace the twisted pair as the primary and
secondary of the transformers are separate and isolated.

Best regards, Richard Harrison, KB5WZI

On Tue, 21 Sep 2004 09:51:59 +0200, Toni
wrote:
Yes, but my main objective for all this is feeding the receiver
with the signal comming from the antenna well above the roof, and
(try to) avoid feeding the noise picked by the feedline on it's
way down. Light dimmers, fluorescent tubes, the computer, TV,
etc. I can notice a noise difference when turning them on/off,
and that's what I want to solve/improve.

Hi Toni,

Often, this problem arrives through the AC connection of your radio,
and not over-the-air. You can test this by removing the power cord,
and running off of batteries.

If your set runs off of AC only (no battery option), try borrowing a
portable radio that covers the same bands and use the same
antenna/ground.

If this last step reduces the noise (with the noise sources ON); or
you cannot obtain a portable radio, then try one more option. Use an
extension cord (so that you don't have to move the radio, or
disconnect and move antenna leads) and plug your radio into a wall
outlet in another room. Move this plug around to see if it impacts
the noise problem.

If you find noise is variable by this technique, you are sharing a
circuit with noisy sources and the noise is coming in through the AC
supply. How you solve this is a function of what choices are
available to you. It may mean moving your listening position, or
buying/building an AC line filter (which should not be
expensive/difficult for a low wattage item). This also presumes that
the AC distribution system has a good ground (not always available).
If this is not available, you may need to do it your self, but this
needs to be investigated with care.

73's
Richard Clark, KB7QHC