eHam Forums => Antennas, Towers and more => Topic started by: KK4SWX on October 01, 2016, 02:08:12 PM

Title: Hygain AV-18VS coil
Post by: KK4SWX on October 01, 2016, 02:08:12 PM

I live in an apartment.  The landlord has been kind enough to grant me permission to put antennas on the flat roof, so long as I do no damage.  I have a Hygain av-18vs that uses a side mounted coil to change bands.  I have found many different replacement coils online.  But I have not found one that is remotely changed.  Can anyone direct me to a means of changing coil contact position, remotely?



Title: RE: Hygain AV-18VS coil
Post by: WB6BYU on October 01, 2016, 06:09:55 PM
The coil assembly from a "screwdriver" mobile antenna is designed to do that:
the coil is rotated using the motor from a cordless screwdriver which slides it
along a contact.

Another approach is to use a roller inductor driven by a motor.  Generally
you also need some sort of remote turns counter, as the coil might have
30 turns from one end to the other.

A third approach is to use relays to select the right tap on the coil.  That requires
finding the proper tap point for each band in advance (usually by trial and error)
then using relays to switch among them.

Note however that the 18VS isn't always properly matched with just a coil in
series the way the antenna is provided.  A series inductor works when the radiator
is less than 1/4 wavelength, but between 1/4 and 1/2 wavelengths it would need
a capacitor instead, and when you are close to a multiple of 1/2 wavelength the
impedance may be high enough that you need some other matching network.
(The series coil on a mobile antenna only works when the total whip length is
less than 1/4 wavelength, which is usually the case on 20m and lower frequencies,
but might not be on 10m through 15m when a long whip is used for higher efficiency.)

If you are mostly interested in 40m and 80m, the coil should work.  For truly multi-band
operation then some sort of relay-switched set of fixed-tuned networks for each band
will probably be better.  The modern approach, of course, is a remote auto-tuner mounted
at the antenna feedpoint.

Title: RE: Hygain AV-18VS coil
Post by: W0BKR on April 15, 2019, 11:00:28 AM
A bit late to the show but I heard of a company Wolf River that sold a varible coil kit to swap out on that antenna that allows you to manually (or electrically - dunno) move the slider up/down the coil.  Is that something that is still available?

Title: RE: Hygain AV-18VS coil
Post by: AA5MT on April 17, 2019, 08:50:50 AM
What you need is a stepper motor driving a roller inductor.  This would be less expensive than buying a screwdriver antenna coil assembly.  You would have to program any controller with memories, which would be very nice.  Reading the frequency from the radio interface would be better, as the antenna could follow the radio.  Pricewise, it might be cheaper to get the screwdriver and interface, as they are in production.