In message , Boomer
writes
On 10/15/2012 6:42 PM, W5DXP wrote:
On Monday, October 15, 2012 2:59:37 PM UTC-5, Boomer wrote:
(A G5RV) is not actually a bad antenna on 20 meters. However, the
performance on 75 is abysmal.
Would you please itemize the losses and radiation patterns that
explain why you believe that to be true for 75m?
--
73, Cecil, w5dxp.com
I don't have any of the math for you. However, I know what works and
what does not. You probably do not listen to your own G5RV. Its poor
performance is pretty clear to anyone who listens to you. However, if
you have an emotional bond with your G5RV, I would not want to be the
cause of any alienation in your relationship. Keep using it. The low
effective radiation makes for more band space for the rest of us. The
signal just disappears into the noise level.
Better yet just put up a random wire and a water pipe for ground. You
can get a perfect match with a tuner. It will work better than the long
wire the guy wanted to bury in the ground :-) In fact I recommend you
try burying your G5RV and make a stealth G5RV. It will still load with
a good tuner.
However, if you were actually interested in improving your transmitted
and received signal, you might tune around 75 and listen to those
stations putting out a big signal. Especially in the daytime when band
conditions are bad. Ask someone with that big signal what antenna they
are using. If they are using a G5RV then you are right and I am wrong.
I doubt if many will choose to use a G5RV when they can put up a
'better' antenna, and it's unlikely that the stations putting out really
big signals are using one. However, subject to an appreciation of its
characteristics and limitations, it can usually be made to perform
reasonably well.
Regarding the use of shorter-than-halfwave dipoles, the following
writings of W8JI (and the included links) are worth a read.
http://www.w8ji.com/short_dipoles_and_problems.htm
I've had a quick look, and it seems that the main cause of poor
efficiency/performance is that as the dipole is progressively shortened,
the increasingly high SWR causes high losses on the feeder. Cunningly
minimise the SWR, and you can still have a reasonably efficient
radiator.
It may also be useful to look at one of G4FGQ's programs - 'Dipole3'. If
you enter zero feeder lengths, this gives a quick assessment of the
efficiency of the dipole alone.
http://www.zerobeat.net/G4FGQ/page3.html
http://www.zerobeat.net/G4FGQ/dipole3.exe
--
Ian