Hi Tom,
I used a CB vertical when I went to YJ0AX a couple of years ago. The antenna
was a CB vertical with a stinger down the side. It will work fine the way
you describe but easier to be open at the top.An article regarding the
antenna is on :
http://www.radioelectronicschool.net/files/downloads/stationmaster-2.pdf
A Hard-Working Garage Sale Antenna
With the decline of interest in 27 MHz CB, "Stationmaster" CB base station
half-wave verticals are starting to show up at garage sales, sometimes even
to be had for the dismantling and removal from homeowners who may have
inherited one with a home purchase. As found, they are a ground-independent
½ wavelength 11-metre vertical. It would be a relatively trivial exercise to
convert one to the 10-metre ham band by chopping a bit off, but there is a
better way to go:
By adding roughly 2 metres to the base end, it becomes ¾ wavelength on 10
metres.
Insulated from it's mounting hardware, and fed in typical ¼-wave fashion,
the impedance is a good match to 50-ohm coax. A 3-metre downrigger of
regular house wire (see
illustration)
allows it to resonate on 15 metres as well, also as a ¾ wavelength.
Three-quarters of a
wavelength on 15 metres equals ¼ wavelength on 40 metres, so a good match
there as well.
With the revised height, it works out to about 3/8ths of a wave on 20
metres. Normally, this would be a bit of a problem to match, but a ¼ wave
transmission line transformer made of
RG-11 75-ohm coax (accounting for velocity factor, of course) transforms the
mystery impedance nicely to 50 ohms.
Four bands (40/20/15/10) from a simple monopole! Not bad for an afternoon's
work and about 20 bucks worth of assorted clamps and fittings and a bit of
ingenuity!
Trent Sampson
[PO Box 1647 Toowoomba Qld 4350] VK4TI
Trent Sampson
[PO Box 1647 Toowoomba Qld 4350] VK4TI
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Tom Osborne
Sent: Monday, 27 November 2006 2:41 PM
To: Towertalk
Subject: [TowerTalk] 15 meter decoupler
Hi all
Is there a way to put a decoupling stub on top of my 20 meter vertical to
make it work on 15? Would I just put, like a 5 foot section shorted at the
top, and open at the bottom, spaced a couple inches? 73
Tom W7WHY
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