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.