[Hallicrafters] RTMA Dummy Antenna needed for alignment?

[Roy Morgan]

On Apr 27, 2008, at 10:55 AM, Randy Perkins wrote:

> Please disregard my posting about the RTMA dummy antenna. With some  
> additional searching the archives, I was able to locate where it  
> looks like I can use an old TV balun and get good results.

Randy,

I have read that SOME TV baluns work fine an HF and some not so well,  
so try any that you have and use the one(s) that give you the  
strongest signal.

You certainly will get a signal through it and the impedance "seen"  
by the radio may be more appropriate, but putting much confidence in  
the number of microvolts you think are feeding the radio with is  
probably unwise.  Certainly, the balun will give you a ballpark  
number to decide if the radio is working right or not.  Don't spend a  
lot of worry over trying to get 50 uV for S-9, however.

I don't remember the values in the standard dummy load, but when I  
read your post, the numbers you quoted seemed a bit strange (I did  
translate to microhenries).  But no matter. The purpose of the thing  
is to SIMULATE the action of a typical wire antenna when it is hooked  
to the high impedance terminal of the older radio sets.  It is  
definitely a broad approximation, and good enough for Guvmint Work!

If you want to have lots of confidence in the real number of  
microvolts at the antenna input terminal, one way is to make a 50:1  
or 100:1 divider whose input impedance is close to 50 ohms.  (49 ohms  
and 1 0hm as a voltage divider, or 50 ohms and half an ohm).  Use a  
signal generator meant to have 50 ohms on the end of the cable, and  
put the attenuator at or very close to the radio input connection.   
No, the impedance presented to the receiver input terminal is not the  
rated value, but you can better count on the voltage you figure is  
present there.

Note:  Typical values for just discernable signal on CW are 5 to 15  
uV on AM band, and 1 to 5 or 10 uV for HF bands.  Signal level to get  
S-9 on the meter can be from, say, 20 to 100 uV.  Some radios can be  
adjusted for the typical spec of 50 uV.  UNLESS it's an SX-42.  Two  
of them I've measured with perfectly fine low signal sensitivity  
needed WAY more than 50 uV to get S-9.  Like many millivolts on some  
bands.  Never did figure out why.  An SX-88 could be adjusted right  
on the money at 50 uV according to the manual procedure.

Roy

Roy Morgan
k1lky at earthlink.net
Lovettsville, VA 20180