Topband: Switching in Extra Antenna Length

[Richard Karlquist]

I have been experimenting with a similar
system for several years.  I have a 90 
foot aluminum tubing vertical with PVC
insulators at 30 and 60 feet.  Thus it is
broken up into three 30 foot sections
that are electrically isolated.  Latching
vacuum relays at the 30 and 60 foot level
short out the insulators to make the total
length 30, 60, or 90 feet.  These configurations
are used for 40, 80, and 160 meters
respectively (there are four 30 foot top
hat wires to tune it to 160).

The biggest problem with this system has been
relay reliability.  I have been using 
Jennings latching vacuum relays.  I have had
to replace the relay at 60 feet four times in
2 years.  The relay at 30 feet has never failed.
I am tired of conducting a science project to
figure out why they are failing, and am simply
going to try conventional relays.

A few months ago, I took the vertical down for
the summer to rebuild it.  I am going to replace
the vacuum relays with big contactors with 
30A contacts and 1/4 to 1/2 inch spacing.  There
will be two sets of contacts in series to increase
the voltage handling (probably doesn't double it
though).  

Another issue is that you have to have either a
choke or a string of resistors across the relay
contacts to prevent static buildup.  Choke overheating
has been a problem.  I am going to change to
a string of high resistance resistors.

The choke at the bottom is no problem because the
voltage is much less.  I run the wires up the
outside of the tubing and put an RF bypass capacitor
between the lines and the tubing at each insulator.
This makes the tubing and the wire act as one
conductor.  Spiral the wire around the tubing
(about 1 turn per 3 feet) so it doesn't flap
in the wind and making annoying noises.

At the 30 foot point, you have to conduct the DC
for the 60 foot relay though an auxiliary relay
or an RF choke.  I used a choke before, but I am
leaning towards an extra relay this time.

I am also going to add an insulator at 42 feet for
the new 60 meter band.  BTW, I use the 60 foot
configuration as a 5/8 wave vertical on 30 meters.

Also, as W8JI recently said, don't use sealed boxes
to hold the relays.  They fill up with water.  Just
use a rain hood.

Despite all the troubles with this system, I still
think it beats using traps hands down.  I revisited
that a few months ago and again decided against 
traps.

Any additional insight on this problem will be
greatly appreciated.

Rick Karlquist    N6RK
richard@karlquist.com
www.n6rk.com
www.karlquist.com