[Elecraft] OT:G5RV thoughts

[N2EY at aol.com]

In a message dated 5/19/07 11:02:07 AM Eastern Daylight Time, n7ws at yahoo.com 
writes:


> www.k6mhe.com/n7ws/G5RV.PDF


This is the 1958 article, which states that the antenna was designed in late 
1945-early 1946, but does not refer to any earlier article. 
> 
> Reading this will reveal the following:
> 
> 1. Varney *did* intend to operate the antenna on
> multiple bands, although 20-meters was the design
> center frequency.
> 

Yup. The antenna was specifically developed for the limited space he had in 
North Buckinghamshire, and he wanted coverage of 160, 80, 40, 20 and 10 meters. 
15 was not a ham band then, and for 160 he tied the feeders together and fed 
it against ground as a toploaded vertical. 

> 2. A version with an all open-wire feeder *is* still a
> so-called G5RV.  Varney even specifies that a version
> *without* the coax is better on some bands.  
> 

Yup. He specifies in the diagram that the feeder should preferably be a 
multiple of a quarter wave at 20 meters. 

> 3. The only thing "magical" about the antenna is the
> choice of radiator length.
> 

I disagree! The choice of feeder length plus radiator length makes it easier 
to match , in either version.

> 4. On 20-meters, the open-wire section of the coax-fed
> version *is not* a "matching section" but an impedance
> repeating section.
> 

Agreed. On 20, the antenna is essentially 3 half waves long, which means a 
feed impedance of 100 ohms or so, nonreactive, depending on antenna height. The 
34 foot openline feeder just repeats that 100 ohms nonreactive to the other 
end.

> 5. Varney repeats an often made error when he states
> that some part or another of the antenna resides in
> the open-wire line, i.e is "folded into the feeder." 

I wouldn't call it a mistake, but it can be a very confusing image. 

What is meant is this: In the case of a resonant dipole, where the antenna 
wire itself is a resonant length, the feedpoint is nonreactive;  that is, the 
feed impedance is a pure resistance at the resonant frequency. If a feedline of 
the same impedance is connected, it will operate without standing waves, and 
can be of any length without affecting the match or impedances.

But with a dipole that is not resonant, such as the G5RV on 80 meters, the 
feedpoint is reactive - the impedance there has both resistive and reactive 
components. If a feedline is connected, it will operate with standing waves. And 
there will be points on the feedline where the impedance is a pure resistance. 
If, at that point, the feeder is connected to a second piece of the correct 
impedance, the second piece will operate without standing waves.

IOW, if the length of the antenna and feedline are chosen correctly, the 
entire *system* will be resonant, and the impedance at the shack-end of the line 
will be easy to match. 

Of course in practice there are lots of compromises because the bands aren't 
in perfect harmonic relatiionships (the center frequency of 40 meters isn't 
exactly twice the center frequency of 80), there are end effects and ground 
effects, etc. 

> If this were true we wouldn't need acreage for
> operation on 160-meters.
> 

Not exactly. A double-sized G5RV could be made - 204 foot flat top and 68 
foot feeder, and the same principles applied. 

---

The thing I find fascinating, at this point, is that while there are many 
references to the antenna having been developed in 1946, nobody has yet found an 
article describing it before 1958. Perhaps it was just passed around by 
word-of-mouth until G5RV got around to writing it up in 1958?

73 de Jim, N2EY


**************************************
 See what's free at 
http://www.aol.com.