[Hallicrafters] RTMA Dummy Antenna needed for alignment?

[Roger D Johnson]

Jim Wilhite wrote:
> The reason I use the 50 ohm resistor is that the manual for my URM 25F 
> states that the generator should be terminated into 50 ohms.  Given the 
> impedance of the antenna coils is higher you will have a bit of a 
> mismatch, but using the 50 ohm resistor will improve the matching.
> 
> Like some people have said, you won't notice much difference whether you 
> use the resistor or not, but I use a dummy load with a coaxial T 
> connector at the dummy load then put the signal generator into one side 
> of the T and the line to the receiver in to other side of the T.  One 
> could sub the antenna for the dummy load with pert near the same 
> results.  It would take a bit more signal to see the meter move, but 
> workable.  One thing some fail to do is disable the AGC, be sure that is 
> turned off or keep the generator output at a low level so the AGC is not 
> working.  You can monitor the AGC line with a VTVM to be sure the 
> receiver is not in saturation.
> 
> Some receivers disable the meter when the AGC is off.  To counter this, 
> use a VTVM across the phone jack or speaker terminals, I have a GR audio 
> power meter which is very handy.  Or one could monitor the output of the 
> detector with a oscilloscope and set for widest pattern.
> 
> With any old balanced input receiver I have never seen a lack of 
> sensitivity if the receiver is working properly no matter how one aligns 
> the antenna coils.
> 
> Jim/W5JO
> 

If you have a 50 Ohm generator, and you want reasonably accurate
measurements, you can use a matching pad. This ensures the generator
is properly terminated and that the receiver sees it designed input
impedance. In addition, you can design the pad for a known amount
of attenuation which will allow accurate sensitivity measurements.
Pads for 200, 300 and 450 Ohms at 20db attenuation should cover most
all older receivers.

Here is a program for designing pads:

http://www.smeter.net/electronics/padmatch.php

Most folks also recommend a pad for straight 50 Ohm systems to damp
out any impedance variations between rx and generator.

73, Roger



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