[ARC5] Rollerductor setting

Thu Apr 20 10:11:10 EDT 2017

Here is my take on the matching:
The ARC-5 and ARA/ATA were all designed to work with short, capacitve antennas as one might expect on an aircraft.The radiation resistance range is spec'ed at 5 to 12 ohms . The roller inductor was there to "tune out" the capactive reactance of the antenna.
I have tested these units with sloping wires. They will load into amazingly short lnon resonant lengths.The official dummy load or phantom antenna (a-61-a)for this series consisted of a 5 ohm resistor in series with a 100 pf cap.This is an extremely reactive scenario. 5-j425 ohms!

If we assume that the antenna load is actually 50 ohms ( and very few are) , a 4:1 unun with the high side to the antenna should provide 
a 12.5 resistance for the Tx to match into. Even If it were a perfect 50 ohms and no reactance you wouls still need to resonate the roller inductance with a capacitor to get the system to load properly.For best results, try to provide the transmitter with a load within it's published design parameters
Since antennas are have variable characteristics, selecting the cap for a center position  while tuned in the center of the band will allow you to compensate for inductive or capactive reactance presented by the antenna as you tune up or down. (assuming the that the antenna is tuned to the band center) . 

I can tell you that a 100 to 150 pf/5kv transmit cap  in series with a 4:1 unun worked very well for me on 80 meters.That being said, your antenna may be be quite  a bit different due to construction , height above ground, proximity to other objects, etc.The best thing to do is to have someone with a an antenna analyzer map your antenna response over the BW. That's the only way you know for sure what to match. I found that mine had issues at the very low end of the band where the impedance was about 100-j75 which the the Tx didn't like at all.Fixing the antenna is another item on the "to do list"  and another reason to say "ASSUME NOTHING".

I have never tried the series cap WITHOUT the unun  because I thought it would be too far out of the specified range.The variometer was designed for the lower impedance, so that may introduce inefficiencies.
How many watts do you get in CW mode?

    On Thursday, April 20, 2017 2:03 AM, Dennis Monticelli <dennis.monticelli at gmail.com> wrote:

 Yes, there will be a series resonance with the variable capacitor.  Sure, just set the rollerductor first and then adjust the capacitor.  If that combo doesn't give you the loading you desire, try adjusting the rollerductor differently.
Dennis AE6C
On Wed, Apr 19, 2017 at 10:11 PM, AKLDGUY . <neilb0627 at gmail.com> wrote:

Hi Dennis
OK, so the rollerductor and the 150pF series cap interact? Is that it?
So I should set the rollerductor arbitrarily to more than half the inductance and adjust the cap to suit?
I like your harmonic reduction comment. It makes sense.
73 de Neil ZL1ANM

On Thu, Apr 20, 2017 at 4:10 PM, Dennis Monticelli <dennis.monticelli at gmail.com> wrote:

Neil,
Whether 80M or 40M, I like to adjust for at least half the rollerductor.  Too little inductance will compromise harmonic attenuation.
Dennis AE6C
On Wed, Apr 19, 2017 at 7:12 PM, AKLDGUY . <neilb0627 at gmail.com> wrote:

To match my T-19/ARC-5 (3-4 Mc/s) to 50 ohm coax, I'll be using a 150pF variable capacitor connected in series with the inner of the coax.
This configuration worked very well with my BC-AO-230 with the cap at about half-mesh.
What setting should I use for the ANT. INDUCTANCE?Frequency is 3.850 MHz for the ZL AM net.
73 de Neil ZL1ANM
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