OCF Dipole Vs G5RV Vs Folded Dipole

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Dene Oehme:
I want to try another antenna for HF work and I'm tossing up between a G5RV(short 80m version), an OCF 80m dipole and a folded dipole like the Diamond WD330/Icom's AH710.

40m (130') is my absolute maximum length and then it will have top partly go over a large steel roof.  I've got about 25metres (82') with nothing under it in my yard.  This is why I'm interested in the folded dipole or G5RV.

I've heard the folded dipoles are very quiet with regards to QRN which is very important to me in my city location.  I've also understand the terminating resistor opposite the feedpoint means some of my TX power is getting converted to heat.
 
The G5VR can cover the bands I want but how quiet is it to QRN compared to the folded dipole.

If it's down to the G5RV Vs the OCF, how efficient is a G5RV compared to the OCF and which is quieter to QRN?

Thanks in advance.

Dene
VK5DO

Dale Hunt:
The resistive-loaded folded dipoles run around 5dB down
from a doublet of the same length as long as it is at
least 1/3 wavelength long.  The losses go up fairly
quickly past that.  The ones that are less than 100' long
will be down perhaps 10dB on 80m:  that is one way to
attenuate noise, but really doesn't help your signal to
noise ratio since received (and transmitted) signals are
down by the same amount.

There are several reasons why an antenna will be considered
"quiet" for noise, and many of them you have no control
over.  Radiated noise will be picked up by your antenna
just like any other radiated signal.  Whether vertical or
horizontal polarization gives less noise depends on your
local noise sources - it will vary.

Noise can also be picked up on the feedline, especialy if
it runs through a houseful of noisy electronic equipment
on the way out to the antenna.  To avoid this you use
the same precautions as you do to keep common-mode RF
from flowing on the outside of the coax when you are
transmitting:  baluns and good balance, possibly with
the aid of RF chokes and grounding as needed.

So if you are putting up a dipole (or something like
a G5RV that acts like one) any option will pick up the
same noise on the antenna.  That's what antennas do,
and there is no magic that can pick up one radiated
signal and not another based on whether it is intentional
or not.  The best you can do is to make sure that your
feedline is properly decoupled from the antenna to
eliminate common mode currents, and that can be a
particular problem with some of the OCFD designs.

If you just want to operate 80m, a simple dipole fed
with a 1 : 1 balun should work as well as any of the
options you mentioned.  For multiband operation, the
OCFD may be usable on more bands without too much loss
in the feedline, but the G5RV with a 1 : 1 balun at
the bottom of the twinlead will likely give less noise
pickup on the feedline if that is where the problem is
coming from.  (It may not be.)

Tom Rauch:
Quote from: VK5FDCO on November 16, 2009, 06:48:15 PM

I've heard the folded dipoles are very quiet with regards to QRN which is very important to me in my city location.  I've also understand the terminating resistor opposite the feedpoint means some of my TX power is getting converted to heat.

You will hear a lot of things, only some of them are true. It is no quieter. It does lose power, sometimes a great deal of power, as heat.
 
Quote

The G5VR can cover the bands I want but how quiet is it to QRN compared to the folded dipole.

They are the same.


Quote

If it's down to the G5RV Vs the OCF, how efficient is a G5RV compared to the OCF and which is quieter to QRN?

Efficiency depends on construction details. Neither is quieter if installed correctly, but if not installed correcty the OCF can pick up more feedline noise.

It depends more on how they are built and what you use than anything. There is no magic. A simple dipole, as BYU suggested, is as good as or better than anything you have mentioned for noise and transmitting on one band.

DAVE CUTHBERT:
WB6BYU and W8JI described the situation clearly. Of the three antennas the OCF dipole should be a bit better on 80 meters than the shortened G5RV. On the other bands I suspect it's a toss up. The OCF dipole will tend to have more feedline radiation than the G5RV and may pick up more local vertically polarized QRN.

As stated the terminated dipole (Diamond WD330) will be down from 6 to 10 dB on 40-10 meters and probably more on 80 meters. This antenna is simple to model using NEC. I can run a sim for you if you'd like.

Given your aversion to QRN and your worry about running the dipole over the metal roof it looks like the shortened G5RV is the winner.

Gunter Lindermeier (Lindy):
Put up a 130" doublet as high as you can. Add a good quality RG6 section of coax from the ladder line to the tuner and read about the simple old doublet here in old posts. Consider a receive antenna!

73 de Lindy

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