[Hallicrafters] RTMA Dummy Antenna needed for alignment?

[Jim Wilhite]

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


----- Original Message ----- 
From: "Edward B Richards" <zuu6k at juno.com>
Subject: Re: [Hallicrafters] RTMA Dummy Antenna needed for alignment?


> OK, Jim;
>
> I understand. You would put the 50 ohm resistor at the generator's
> output, before the coax to the receiver. Actually, most hi end 
> generators
> have a 50 ohm output. I know my UG25F does. I agree that too much 
> energy
> is wasted on impedance matching. Actually, the best way is to connect 
> the
> receiver in it's final configuration and peak the antenna coils to a
> station or just peak the noise. No big deal. Good luck.
>
> Ed  K6UUZ
>
> On Mon, 28 Apr 2008 22:16:44 -0500 "Jim Wilhite" <w5jo at brightok.net>
> writes:
>> I forgot to mention that Ed.  However my gen is high impedance so the
>>
>> resistor is what the input circuit sees.  I don't think many people
>> have
>> signal generators with 50 ohm output, but I may be wrong.
>>
>> Jim/W5JO
>>
>> ----- Original Message ----- 
>> From: "Edward B Richards" <zuu6k at juno.com>
>>
>>
>> > Jim;
>> >
>> > You don't say where to put the resistor. If the generator has a 50
>> ohm
>> > output impedance you don't want to put it across the receivers
>> antenna
>> > terminals as you would be double terminating the sig gen. The
>> radio
>> > already is a termination. If you are going to be using 50 ohm coax
>>
>> > feed
>> > line, just peak the antenna coils into the generator. That will
>> > optimize
>> > the radio for a 50 ohm input.
>> >
>> > Ed  K6UUZ