[ARC5] BC-AS-230 Transmitter Follies, Part 1

[Ben Hall]

Hi Mike and all,

On 12/31/2014 3:52 PM, Mike Hanz wrote:
> Precisely.  As Robert Downs mentioned on 12/22, it's why you use a
> phantom antenna instead of a dummy load to tune up these sets.  Xc of
> the A-55 dummy load at 7MHz=-j223 ohms.  If Xl of the "proposed" .11uH
> inductor is +j5 ohms, then obviously the cap swamps the inductor even at
> ten times that inductance.

My thoughts were along these lines - the manual notes that if you don't 
have an A-55 phantom antenna, one should substitute a 5 ohm, 10 watt 
resistor in series with a 100 pF capacitor.  The most common, easiest to 
find resistor would have been wire-wound...and the manual doesn't 
specify the use of a non-inductive resistor.  So...I just grabbed parts 
out of the junk box and slapped one together.  ;)

Interestingly...the manual does give us some recommended antenna 
configurations:

"A transmitting antenna which fulfills the requirements outlined above, 
in the 6,200-7,700 kc frequency band, consists of a T structure having
a flat top section which is between 16 to 18 feet long, with a down lead 
about 9 feet long to the lead-in insulator. If an L antenna is used, its 
total length from the lead-in insulator to the end of the top section 
should be 20 to 25 feet."

For kicks and grins, I modeled both a "T structure" and an "L" antenna 
per the lengths above in EZNEC.  At over 100 feet off the ground, at 7 
MHz, the "L" antenna was 4.8 -j20430 ohms and the "T" antenna was 3.7 
-j20300.

Those five-digit -j's above don't seem right.  But...the real portions 
are pretty close to the 5-ohm resistor called for.  And really confirms 
what we already know - the sets were not designed for 50 ohm antennas!  :)

(Full disclosure:  imaginary math isn't my thing...and my knowledge of 
the use of EZNEC is limited...so I don't know if the five-figure -j's 
above are reasonable or are due to something I screwed up.)

Interestingly, I dug out the AA-908 Antenna Analyzer and stuck it onto 
my "phantom antenna."  At 5.5 MHz, which is about where I was testing 
the 230 transmitter, it measures 2 -j125.  It is worth noting that the 
AA-908 isn't known for high accuracy when you get far away from 50 ohms 
pure resistive.

thanks much and 73,
ben, kd5byb