nbr wrote:
On Sun, 27 Jul 2003 19:50:04 -0500, W5DXP
wrote:

nbr wrote:
1) How to feed the antenna and be able to run legal limit, all bands?

Put your matching network at the base of the antenna. Feed your matching
network with coax.

Thanks for your reply to my message, but this is not feasible. The
antenna is 120' from the shack, unblanaced transmatch in the shack.
Looking for matching network at the base of the antenna (or apex,
maybe), perhaps balun (???), but needs to be broadband so antenna can
operate multiband.

Cecil's right - it can't be done without a matching network at the base,
because the feedpoint impedance varies too much between bands.

Your antenna is about a twice-size version of mine. I have a 35ft
vertical, with a 35ft horizontal section followed by a "switch" that can
connect another 65ft (all dimensions approximate). It's fed through
matching networks at the bottom of the vertical, against a large number
of radials.

The extra length of horizontal is only used for Top Band
(quarter-wavelength, low Z, needs those radials) and for 80m short-skip
(half-wavelength, mostly horizontal, voltage fed so doesn't use the
radials much).

With the "switch" open, it's simply a 35ft vertical, 35ft horizontal
inverted-L.

Top Band: 1/8-wave, inefficient but long enough to be usable for easy
QSOs.

80m: bent quarter-wave, surprisingly effective for DX, but low-Z feed
and needs those radials.

40m: voltage-fed half-wave, half vertical and half horizontal - from the
UK it's good for mixed European/DX working, but obviously receives the
European signals even when you don't want them. High-Z feed.

Higher bands: too long - wastes power in lots of little lobes pointing
in useless directions. Gets worse and worse as the frequency goes up,
and even a simple dipole is better. Mostly high-Z feed.

As Cecil says, there's no choice but to feed it at the base through
matching networks on most bands. Trying to feed it from the shack end
through the long run of coax is a non-starter, except on the bands where
it's a low-Z feed. On all the other bands the SWR is too high and the
losses in the coax are crippling.

For many years I used a conventional T-match ATU out there in a
waterproof box. Band-changing was possible, but a chore. Then I tried
custom switchable networks, but that became too complex. At present I'm
using a military auto-ATU that is 1kW rated and works like a dream.

I would certainly recommend the 35+35ft along inverted-L with an
auto-ATU for any small backyard. The extra horizontal length is fine if
you can get it - and the advantage of using the auto-ATU is that it can
be *any* practicable length; you don't have to worry about matching any
more. The only problem is that amateur auto-ATUs are limited to medium
power.

Your system is twice as big as mine, so you can shift all the above
comments down one band. It should go like a rocket on Top Band, but even
on 40m it's getting over-long. If you have other antennas for the higher
bands, think of the inverted-L only as your "low-band special" - then
two or three remotely switched, pre-tuned matching networks will be all
you need.

In your situation I would also consider a remote switch at the top of
the vertical, to disconnect the horizontal part completely. You'll then
have a very effective DX antenna for 40m. Take a look at N6RK's site,
which has some information about switching out the upper sections of
antennas for the bands where they are "over-tall".


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek