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