Topband: RX 4 square

[Rick Karlquist]

Tom Rauch wrote:
>> birdies were not present on the transformer version.  I
>> suggest to anyone
>> who does not believe this: try it yourself.
>
>
> I have, several times and the results were not good. It

I got good results with Mini-Circuits transformers.
Have you tried those?  Did you use a preamp after the transformer?

> At quiet locations a 25-30dB gain amplifier with a few dB
> noise figure and 3000 ohm impedance amplifier won't get you
> into atmospherics. This is especially true in a  small array
> with close spacing, since the signal from the elements don't
> add in phase. For example at my location I barely get into
> propagated background noise with an active element 10 feet
> tall.

We're all envious of your location.  Most locations have
substantial local noise and BCB QRM.

>
> Leaving 25-30dB gain out of a system that often needs even
> more gain isn't a good idea. Of course if the element is
> long enough then the rest of the system can have loss
> without problems. That's a heap of gain to make up for in
> the shack!

Assuming a 3000 ohm antenna, there is virtually no difference
between a unity gain voltage follower with 3 dB NF and a
combination of a 36:1 impedance transformer to 75 ohms followed by a
1 dB NF preamp in either gain or signal to noise ratio.
Noise figure is invariant through a transformer.
In a very quiet location, lowest NF always wins.
The transformer will have a voltage transfer ratio of 1/6,
or -16 dB. If a 22 dB gain preamp is used in front of the
radio, the resulting total gain will be 6 dB higher than
than the active case.  The preamp gain will exceed the NF
of my FT1000D by 10 dB, which is about right.  There is
nothing magic about an amplifier with high input impedance
and low output impedance.  It still has a NF that determines
sensitivity (at least in a very quiet location).


>
> The other thing the amplifiers do is have a source impedance
> of about 60 ohms or so to drive the 75 ohm cable. A
> reasonable and stable (over frequency) output impedance is
> necessary to maintain phase in the elements. As I pointed
> out earlier the elements can be very reactive (capacitive)
> or they have to appear to be a broadband resistance, either
> is OK.
>
>
> 73 Tom

If the input impedance of the combiner is 75 ohms, whatever circuit
is at the antenna sees 75 ohms, and the coax effectively disappears.
I can't think of a reason why a combiner can't be built with a good
match, but if necessary, 75 ohm buffer amplifiers could be located
at the combiner input.  I can't think of a case where you ever need
a remote amplifier unless the coax loss is a problem.

Rick N6RK