[ARC5] Rollerductor setting

[AKLDGUY .]

> 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.

Yes.
Dave Stinson has previously recommended a series cap of 75pF. I chose a
variable 150pF
(it happened to be on hand). This worked well with my BC-230 transmitter.

Some readers may remember a discussion a while back about a suitable
antenna for my
yard. The antenna that worked well is a very short dipole with the legs
close to 90 degrees
to each other. The far ends of the legs are tied to the fence of my yard at
the two corners
and are about 5 feet high. Close to the sliding door entry to the flat is a
plastic clothesline.
A large old-type glass insulator is tied to the clothesline and the two
legs come through the
eyes of the insulator about 3" apart. From there, they come in through the
sliding door 3"
apart as a sort of open wire feedline to the MFJ antenna tuner's open-wire
terminals.

With that antenna and the 150pF series cap about half meshed, I was able
to tune the
BC-230 transmitter on 3.850 MHz for a very good 1:1 match and 4 watts of
forward power.

I will use the same setup with my T-19/ARC-5. I will not need to tune up
and down the band;
the AM net on 3.850 MHz is the goal for now.

73 de Neil ZL1ANM



On Fri, Apr 21, 2017 at 2:11 AM, J Mcvey <ac2eu at yahoo.com> wrote:

> 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?
>
>
>
>
>
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