Jim,
Perhaps you can explain to me why there is a propensity or misguided desire for
hams to use antennas designed for the 2M ( or 150-160 MHz) range as radiators
of RF energy at frequencies in the 440 MHz range.
Are there no people that realize that even though at 3X the designed
fundamental operation frequency antennas have a substantial pattern break-up
which results in diminished radiation in the desired direction, as indicated in
your comments below of some 10 dB less, plus not considering the phase shift in
the combining/matching harness in which will be a nightmare to achieve a
realize able impedance match, not to mention the loss in benefits of the
positioning of the dipole to the mounting pipe on exposed dipole antennas.
Use of a fiber glass co-linear designed for 150 MHz at 440 MHz is a worse
disaster.
Or use of a 450 designed antenna at 2M.
Why use an antenna with a 10 dB loss ? Better to use a radiating coax on the
tower leg and spare the expense of the poor antenna performance if you want
multi-band operation.
Even those so-called 2M - 450 mobile antennas are no bargain as the
manufacturer does not dare publish the radiation patterns as no one would buy
them if they knew of the poor performance. Does any one really believe there is
no radiation from the 2M portion of a multi- band antenna when operated at 450
? Or no radiation from the 450 portion of the antenna when operated at 2M? And
what is the phase relationship of the radiation on one band to another ? And
how does it affect the radiation pattern of the other ?
WA9ZZU
--- On Fri, 11/28/08, Jim Brown [EMAIL PROTECTED] wrote:
From: Jim Brown [EMAIL PROTECTED]
Subject: Re: [Repeater-Builder] Replacement Dual-Band Antenna..
To: Repeater-Builder@yahoogroups.com
Date: Friday, November 28, 2008, 8:03 AM
You may find that a lot better operation will be had with a VHF antenna used as
a VHF/UHF dual band antenna. The 1/2 wave VHF dipoles are 3/2 wave on UHF and
a current feedpoint is the same point on the dipole on VHF as it is on UHF.
My experience has been on a DB-224 type antenna, but I suspect that other
exposed dipole co-linear antennas would work the same.
Someone here on the group published a pattern on a vertical array of dipoles
operated on the third harmonic with the wide spacing that would be in effect,
and it showed many lobes in the vertical pattern. But it did have one of those
lobes at the 90 degree broadside elevation to the antenna, and other folks
reported that they had operated the antenna this way.
I used a DB-224 as a 440 repeater antenna with a 2 meter remote base diplexed
into the same antenna and was able to use it successfully for several years.
The range of the 20 watt 440 repeater was about the same as the 2 watt 2 meter
transmitter on that same antenna.
73 - Jim W5ZIT
--- On Thu, 11/27/08, Mark Christian [EMAIL PROTECTED] com wrote:
From: Mark Christian [EMAIL PROTECTED] com
Subject: [Repeater-Builder] Replacement Dual-Band Antenna. .
To: Repeater-Builder@ yahoogroups. com
Date: Thursday, November 27, 2008, 4:34 AM
Kevin,
Hey fellow trustee. My name is Mark (KB6SRT) and I am the trustee of two
repeater systems in the Southern California area. This is just a note to let
you know that when we were strapped for money in our early years and we only
had the uhf antenna, I used a Dodge-Phelps 8 bay, dual-eared folded dipole
array for dual band operation and it worked well until we could get a decent
dual band antenna. Though it was a uhf antenna, it resonated on vhf well
enough and had decent SWR to allow regular use.
The really nice thing about this dual-eared dipole is that it had folded dipole
on BOTH sides of the pole (dual-eared) so the coverage was quite impressive and
the input power was up to 250 watts. So any loss in power was made up for with
an amp.
Just a suggestion that it doesn't HAVE to be a dual band antenna. Try
resonating vhf off a uhf antenna.
Hope all works out for you and Happy Thanksgiving!
-- Mark Christian KB6SRT --
Trustee. - FCARA
146.610 (-) pl 103.5-Echolink Node-44576
445.760 (-) pl 103.5-IRLP Node-3952
[EMAIL PROTECTED] com
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