[AMRadio] LINK COUPLING)

[Donald Chester]

I had pretty good results tuning my 80m dipole to 160. I had to add a
switchable extra 60' of OWL, because on 160 the line terminated midway
between a voltage and current loop. I was able to load it by tapping down on
the coil, but the 7 kv variable capacitor would arc over at 100% modulation
running only 100 watts. Adding the extra section of line allowed me to
parallel tune, and it is now almost bullet-proof. I have never been able to
make it arc over no matter how much power I was running. I used it before I
got my radials installed back in 1981-82; I never even bothered with trying
out the vertical without the radial ground system. With the dipole I was
able to work all over north America and got good signal reports. Of course,
it helped that the dipole averages 110'  high.

When I got the radials installed, I noticed right away that the  vertical
was head and shoulders above the dipole, except semi-locally. I used to talk
to a couple of hams in Nashville on 160. They always told me that the dipole
was 30 dB stronger there than the vertical, and made the difference between
hearing a lot of electrical noise mixed in with my signal, and full quieting
with no noise in the background. Within 10 miles or so, the vertical straps
out the noise, but there is a substantial skip  zone between 20-30 miles and
somewhere beyond 100 miles. I still occasionally use  the dipole on 160.

Brian, that smaller capacitor looks like the ones I use here in some of my
coupling units in the shack. Mine are 900 pf/section, rated at about 2500
volts, and one is 1000 pf/section with slightly closer spacing. The 900 pf
ones came out of the military surplus BC-339 RTTY/CW transmitter. They were
used as a loading capacitor in a balanced pi-network for a pair of 833As in
push-pull. I still kick myself to-day because back in the late 60s while I
was in Africa, I stripped the goodies out of five of those transmitters that
had been hauled to a scrap yard, before the locals got a chance to scrap
them for metal. But I only sent home one of those capacitors, because being
solid brass they were very heavy and I was too damned cheap to spend the
money to send them all back to the states. The other four got  pulverised
with a sledge hammer and sold for scrap metal. Something else I hated to
leave behind was the cabinets, which  are each about the size of a typical 1
kw broadcast transmitter, but no way I could have had anything like that
shipped 10,000 miles across the planet without taking out a mortgage.

I did manage to find another one of those transmitters a few years ago, plus
I came across another capacitor at a hamfest.

I don't always use a series or parallel capacitor with the link. For a
single band tuner, I sometimes carefully adjust the number of turns on the
link by trial and error to get a match with my low impedance line. Right
now, I feed my "links" with 440 -ohm OWL, acting as a matched transmission
line, and adjust taps on the coupling coil until I get a match. I check SWR
on the balanced  line with a re-built MicroMatch (see April 1947 QST)
modified to work on 160 as well as the other bands. On 75, the coupling link
has about 12 turns. On 40 and 160, I tap directly across a few turns of the
main coils without a separate link.

My very first link coupled tuner used the same 4-turn swinging link at the
tank coil in the transmitter for each band, and each one of the tuner coils
had a fixed 4-turn link. This worked beautifully on 160 through 20, by
dipping the plate tank, adjusting the antenna tuner for peak coupling with
the link  pretty far out of the coil, then loading up to full plate current
by moving in the swinging link, and finally rechecking both tuned circuits
for resonance. That was back when my antenna tuner was just a few feet away
from the transmitter. The links were connected together using heavy duty zip
cord with the  conductors ripped apart from each other.


Don k4kyv