[Elecraft] dipole antenna efficiency

[N2EY at aol.com]

In a message dated 1/4/08 4:34:29 PM Eastern Standard Time, k0pp at acninc.net 
writes:


> I've never understood the popularity of the G5RV.  It's
> achieved a sort of "cult" following.

It's an ingenious compromise antenna for several bands, that's all.
Actually, just a ~102 foot dipole with a matching system that gives
"low" (but not unity!) SWR on several HF bands, so that a simple
ATU can match it.

  The original design
> 
> was for a 20M -ONLY- antenna, 

Not true!

I have PDF's of the original articles by G5RV himself, and from the very 
beginning it was a multiband design. He had a small garden ("back yard" to us 
Yanks) and wanted to get on the air easily, quickly and simply. For his 
application, it worked.

But it must be remembered that when G5RV designed the antenna, the ham bands 
were somewhat different than today. 30, 17 and 12 meters weren't ham bands at 
all. 9-/75 meters in G land was 3.5 to 3.8 MHz only, and 40 meters was 7.0 to 
7.1 MHz.

Most important of all, the rigs in use were capable of matching "reasonable 
levels of SWR" - meaning 3 or 4 to 1 wasn't considered to be worth worrying 
about for the bands and short lines being run.  

and somehow has
> 
> morphed into a do everything hoax.

Not a hoax, but there's a lot of misunderstanding about the antenna.

The biggest misunderstanding is that too many folks expect to put up a 
G5RV-like antenna and get 1:1 SWR on all parts of every HF band from 80 thru 10 
meters, automatically. And work the world with the same ease as folks with big 
aluminum. 

That's just not going to happen. It's just a dipole with an ingenious feed 
system, not magic.

> I do antenna talks at conventions and hamfests, and
> I always ask; "How many of you use a G5RV"? The
> hands go up and it's usually about 50% of the audience.
> I say; "Gosh, I'm sorry", and try to show them how much 
> easier and more efficient it would be to simply use the
> open wire feeder portion of the antenna and a balun at/in
> the (required) tuner.
> 

It's easier to do it that way *IF* you can make the feedline and antenna 
length such that the balun doesn't have to deal with very high, very low, or 
highly reactive impedances on the bands of interest. Or if you can run the balanced 
line all the way to a true balanced tuner that can handle the impedances 
presented to it. 

> Some years ago the "Carolina Windom" had the same
> sort of following....
> 

And for the same reasons - with the same limitations.

The G5RV and Windom antennas can be useful solutions in many cases. The main 
thing is to understand how they work and what their limitations are.

---

Comparing HF antennas can be very misleading because of all the vagaries of 
propagation and expectations. For example, suppose two hams with identical 100W 
output rigs put up identical dipoles, but Ham A's dipole has a feedline/tuner 
system that is 88% efficient and Ham B's dipole has a feedline/tuner system 
that is 22% efficient.  

Ham A loses only 12 watts in the feedline/tuner system - that's about as good 
as it gets on HF. Ham B loses *78* watts in the feedline/tuner system - 
almost six times the loss of Ham A! 

Yet at the receiving end, the difference is only one S unit - 6 dB. Slight 
differences in propagation could easily mask that and make Ham B's antenna 
appear to be as good or better than Ham A's.

A lot of hams will say a particular antenna "works great" for them. But what 
does that really mean? I remember one multitransmitter Field Day, some years 
ago, when a variety of antennas were tried out by the various station teams. 
All reported their setups "worked great" when notes were compared a few hours 
into the contest.

But for one team, that meant they were able to average 40-60 QSOs/hour, and 
for another team, it meant 10-15 QSOs/hour!  Their expectations were completely 
different. (And compared to truly competitive setups, neither was a 
world-beater).

73 de Jim, N2EY


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