|[AMRadio] new antenna (again).|
hbco2 at sbcglobal.net
Sun Mar 11 14:20:38 EST 2007
----- Original Message -----
From: "Brett gazdzinski" <brett.gazdzinski at verizonbusiness.com>
To: "'Discussion of AM Radio in the Amateur Service'"
<amradio at mailman.qth.net>
Sent: Saturday, March 10, 2007 4:46 PM
Subject: RE: [AMRadio] new antenna (again).
> Yes, I worked on it most of today, the 80 meter
> portion was ok for the most part, the 40 meter section was acting
> crazy, and kept getting tangled up with the 80 meter part.
This form of antenna is a bit unwieldy. The 40 meter element must be
located below the 80 meter element. It helps if you have separate guy lines
for the two elements. It also helps if you match the tenson in the wires,
but the antenna can still twist. Once it is up, however, it is stable.
> SWR at 7290 was 5 to 1. That is bad.
> I added lengths to the end of the 40 meter part but
> it just acted nuts, the 80 and 40 meter segments were interacting..
The 80 meter element should behave as a regular dipole, but the 40 meter
element (and any highter band elements) will interact. Typically the
shorter elements should be cut about 10% longer than the formula.
To tune a dipole, start with the 468/frequency formula. Cut the wire a
little long. Install the antenna an measure the frequency with the lowest
SWR. Use this frequency and the known length of the antenna to calculate a
new constant. New constant = length * measured-frequency. Then use the new
constant and the design frequency to calculate a new length. Cut the
antenna to the new length and repeat. It shouldn't take more than one or
two tries to obtain resonance at the desired frequency.
You may not achieve perfect 1:1 SWR on the shorter elements, and the SWR
bandwith of the shorter elements will be much smaller. However, an antenna
tuner can be used to clean up the mismatch, and the antenna will work very
One way to reduce interaction is to separate the 80 and 40 meter elements.
In the extreme, and If you have room, separate the 80 and 40 elements
horizontally 90-degrees from each other (as in the form of a cross).
> I gave up and took it down, cut the 40 meter section off,
> and changed to open wire line feeder from the antenna to rooftop.
You made a different antenna. It is no longer a classic dipole. A dipole
is two extended wires of resonant lenght. It has a feedpoint impedance that
varies between 40-120 ohms depending upon height above ground.
> So I had 60 foot each side and about 40 feet of open wire line,
> and the heath kit antenna tuner arced at high power
> under modulation on 80 meters.
Low antenna resistance. High degree of mismatch between the feedline and
the radiating wire. The antenna is likely hard to tune and problaby will
have poor performance.
> I tried adding 30 feet of open wire line, no improvement.
> I then cut the antenna down to 50 feet each side, with 40 feet of
> open wire line (roughly G5RV lengths) and it works fine again.
The feedline is not acting as feedline but acting like it is part of the
antenna. There could be signal cancellation.
> I will have to think about what to try next.
> > -----Original Message-----
> > From: amradio-bounces at mailman.qth.net
> > [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Bill Smith
> > Sent: Saturday, March 10, 2007 12:26 AM
> > To: Discussion of AM Radio in the Amateur Service
> > Subject: Re: [AMRadio] new antenna (again).
> > Hi Brad,
> > It sounds like you had the ideal antenna to begin with.
> > There is nothing
> > wrong if the antenna is a bit "long." Short antennas'
> > radiation reisistance
> > takes a nose dive.
> > I've used a similar antenna for years and have enjoyed great
> > performance.
> > 73 de Bill, ab6mt
> > hbco2 at sbcglobal.net
> > ----- Original Message -----
> > From: "Brett gazdzinski" <brett.gazdzinski at verizonbusiness.com>
> > To: "'Discussion of AM Radio in the Amateur Service'"
> > <amradio at mailman.qth.net>
> > Sent: Friday, March 09, 2007 6:26 PM
> > Subject: RE: [AMRadio] new antenna (again).
> > >
> > > I took down the alpha delta dx-dd and the homebrew G5RV, and put
> > > up an 80 and 40 meter dipole fed with the same coax.
> > >
> > > Keeping them from twisting up was a chore, what with them getting
> > > snagged in the smaller trees on the way up.
> > >
> > > Anyway, I got it up and put the 756 pro into it, and tuned
> > > across 80 and 40 meters, resonance was at 3750 and 7176, about
> > > 1.2 to 1 swr, and broad banded...
> > >
> > > So I cut a foot off each end of each dipole, and got
> > > 3887 and 7500!
> > > Quite a jump from 7176 to 7500!
> > >
> > > Tomorrow I will lower it and solder on some pig tails to
> > > the 40 meter section that I can trim without redoing the
> > > entire antenna.
> > >
> > > Seems to work well, on receive anyway, and I will like
> > > changing bands without changing antenna's or using a tuner.
> > >
> > > The 80 meter section goes through the trees and bends down
> > > a little, but I used insulated wire.
> > >
> > > Brett
> > > N2DTS
> > >
> > >
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