[TowerTalk] remote tuner Vs lumped constants (K8RI)

[Jim Thomson]

Date: Wed, 24 Jul 2013 13:15:31 -0700
From: "Rick Karlquist" <richard at karlquist.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] remote tuner Vs lumped constants (K8RI)

> I have several 160 and 75 meter antennas.  I'd like to be able to cover
> most og these bands with the least involvement from the shack after the
> initial set-up.
>

What I do on 40, 80 and 160 meters is to tune the antenna for a
match at the bottom of the band, and then use three relays to
switch in combinations of a binary series of capacitors.
For example, an 80 meter inverted vee is tuned to 3.500 MHz
by trimming the wires.  A series string of 470 pF, 1000 pF,
and 2200 pF is inserted in series with each leg of the inverted
vee.  Each capacitor is shunted by a normally closed relay.
The two 470 pF capacitors are on one DPDT relay (used as a DPST).
Similarly, the other two relays handle the 1000 and 2200 pF pairs
respectively.  The relays can be individually energized to
unshort a capacitor pair and place it in the circuit.
The eight combinations cover the whole 80 meter band
(3.5 to 4) in eight subbands.  There is also a 40 meter inverted
vee physically at right angles to the 80 meter vee.  The two
vees are electrically connected in parallel  The capacitors
also tune the entire 40 meter band easily.  There originally
was a 4th relay that disconnected the 80 meter vee when using
40 meters.  I observed that this relay made absolutely
no difference, indicating that the 80 meter vee doesn't interact
with the 40 meter vee.

Note that the relays must be at the feed point, not in the shack.

The capacitors are CDE MC series and the relays are Omron
MJN series, both available from DigiKey.  Works at full legal limit.

Rick N6RK

##  Very clever !   I did it the other way with 3 x DPST relays and 6 x coils, 
3 per leg. Then ant cut for the high end of each band.   How big are
the CDE  MC series if caps ?  Are they mica ?   They would have to handle
5-6A of rf.   And with 3 x relays, you end up with a total of 8 segments. 

##  watch out though. On any loaded ant, like my half size 80m rotary dipole,
the feedpoint Z is way below 50 ohms.  Matched with a helical hairpin. But
coils OR  caps will be on the lower Z side of the hairpin...so the current through
em in that case is a lot higher.   I thought of using my motor driven vac cap instead of
fixed caps  + relays.   In the end, I opted for a pair of compressible 6-12 uh per leg
tubing coils.  That concept works great  except the min uh is never zero. 

## compressible tubing coils like used in the tornado units  made by seco systems  all
have a 2-1  min to max ratio..  Like  2-4 uh,  4-8, 10-20 etc.  Bottom line is, with
the 6-12 uh coil in each leg, swr is flat from  3375-4100.  But the coils are part of the
loading in my case, and the capacity hats form the other loading.  IE:  I need 6 uh per
leg just to resonate at 4100 khz. 

##  the compressible coils are not practical on any full size ant, the min UH works against you.
You would have to resonate the ant  way above the top of the band..like  4.6 mhz. 

##  so folks have options....and all it takes is a few caps  + relays.... or a few coils + relays. 
Either method works..and sure beats a  coax to coax tuner in the shack.  It will work on a 
40m yagi too. 

Later...... Jim  VE7RF

do what yo