[ARC5] ARC-5 Output Spur Tests- Some Figures.

[Mike Hanz]

Neil Barnett wrote:

>...the harmonic level measurements look very good indeed...
>

Yes they do - and it's even more remarkable considering it was measured 
with a series 50pF cap - as Dave stated, not really an optimal value to 
complete the transmitter tank network properly.  Using that 50pF cap in 
the antenna relay is a huge improvement over nothing, but it was 
actually intended only to permit loading for electrically long aircraft 
antennas that might be used with the command transmitters, ostensibly on 
an emergency basis.   See 
http://members.cox.net/aaf-radio-1/Aircraft_Antenna_Design.html for a 
discussion of typical antenna characteristics, including a nomograph for 
calculating the long wire driving point characteristics on your 
airplane, or Airstream trailer, or whatever.  As Dave correctly 
observed, 50pF isn't the total answer.  My own T-19 needs close to 200pF 
in series with it to eliminate reactance looking back toward the 
transmitter output terminal, which simply shows that each transmitter is 
somewhat different.  In the pursuit of perfection, I ended up with three 
different series capacitors for the T-17, T-19, and T-22 in the rig, 
switched with relays that are paralleled through the wiring harness with 
the "transmitter select" switch on the C-29.  The three caps, relays, 
and the unun are mounted behind the rig in a black wrinkle junction box 
so that the normal antenna daisy-chaining that the command transmitters 
traditionally utilize isn't altered.  I don't use the cap under the 
antenna relay.

>My own view is that an "un-un", like any of the balun
>varieties, will only do a decent transformation if both
>sides face purely resistive terminations.
>Any problems arising from reactive terminations are not
>too serious if the balun is air-wound, but in the case of
>ferrite cored baluns, reactive terminations lead to high
>losses (overheating) of the core and permanent damage.
>

You're absolutely correct, Neil.  I was remiss in not mentioning that 
*/if/* you decided to use an unun, the series capacitor should go 
between the transmitter and the unun.  Doing it the other way around 
promotes the result you described, though ICAS usage and selection of a 
good toroid core certainly reduces the problem.  Some fiddling at the 
antenna end is needed as well.  My attic "antenna farm" requires a 
remote tuner for the HF antenna to get it conditioned to efficiently 
ride the 50 ohm Heliax to the basement.  Since I have nothing but WWII 
aircraft transmitters in the rig, it would be nice to find 75 feet of 
maybe 12 ohm coax and I could do the transformation once...anyone have 
any to spare?  :-D

73,
Mike de KC4TOS