[AMRadio] new antenna (again).


Bill Smith hbco2 at sbcglobal.net
Sun Mar 11 13:50:55 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 4:57 PM
Subject: RE: [AMRadio] new antenna (again).


> Bill,
>
> The ARRL antenna book as lengths to use, but only on balanced
> antennas and tuners. They say grounding one side
> introduces to many variables, its cut and try with coax.

The formulas in the ARRL antenna book (and from various other sources)
specifiy antenna length based upon an ideal antenna in infinate space.  In
practical antennas, ground effect is probably the largest influence, but
surrounding structures can also have an impact.  Grounding one side of the
antenna changes the antenna pattern slightly, but with all the other
influences on the antenna, unbalancing the dipole probably has little if any
overall impact.  The main effect is RF current can travel down (and radiate
from) the outside of the coaxial cable.  It can generally be discounted, or
you can install ferrite beads over the coax at the antenna feed point.  You
will find this described as a "current balun" in the literature.

In amateur service, where operations cover a wide frequency range, antennas
are not necessarily operated at their resonant frequency.  Antenna tuners
are used to match the transmitter to the load (which is the antenna and feed
line), but the antenna may still be operating off-resonance.  When the
antenna is operated off-resonance, it no longer presents a resistive load to
the feed line, SWR increases and the feed line radiates.  While it is very
important to couple RF from the transmitter to the antenna, "high" SWR
(within limits) is not necessarily a bad thing.  The goal is to set up the
antenna and feed line as a system to couple energy to the atmosphere.

It is very important to raise the antenna to its full operating position
when you make SWR measurements.  The SWR of a wire antenna will change with
height, especially when the antenna is less than 1/4 wave height above
ground.

73 de Bill, ab6mt
[wrong Bill]


>
> As it is, with the G5RV, I have only about 30 feet of RG214 coax
> in line, so high swr will not cost me much.
>
> If I was to ADD coax to get away from a high voltage point, that
> is only going to increase the loss in the coax.
>
> At least that's the way I see it...
>
> Brett
> N2DTS
>
> >
> > Brett,
> > Heard you thursday loud and clear in northwestern Mass.
> >
> >  When I had my station setup down in PA my dipole was a
> > similiar 75/40 double wire from a single coax with half the
> > 75 flattop along the roof peak and then zigged down to a
> > garage roof and zagged back up to a window ledge.
> >  The 40 was flat on top at an angle and a single slope to the
> > other corner of the garage roof.
> >  Did have some rf in the shack being under the wire but it
> > was no closer than 25 feet at any point.
> >
> >  The problem with single feed coax to a dual 40/75 wire is
> > that the coax length does not work out for both bands.
> >   Somewhere I found a chart or formula for figuring this out
> > and there is one length that is close.
> >   Resonance was checked with the wire as run then I added a
> > piece and checked again but found removing a similiar length
> > got it about in for 75 with 1.5 or so SWR with 40 just a bit worse.
> >
> >   Does anyone here know where that coax length table was?
> > Might have been an Antenna Hndbk or regular Handbk or online..
> >
> > Bill, KB3DKS/1
> >
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