Topband: RX 4 square

[Tom Rauch]

> 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 
would also be interesting to see how others who have built 
arrays weigh in on this. I know ZL3IX and others use active 
elements.

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.

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!

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