Measuring Feedline Impedance

[VK2CSW]

Greetings

I am looking at trying to "tune" a G5RV style antenna. A part of this will involve adjusting the ladder line feeder. It get a real value for the impedance of this cable would the following process work?

- Terminate one end at 50 ohms
- Haul the line up off the ground
- Connect the other end onto a VK5ZVS Antenna Analyser
- Tune the Analyser to 14.15Mhz
- Read the impedance

Also in the above configuration, would a low VSWR indicate a "tuned" matching section?

Regards

Colin
VK2CSW

Edited coz I can't spell

[VK5ZD]

Hi

Assuming that the ladder line impedance is not 50 ohms (which isn't likely), then the analyser will read 50 ohms only when the feed line is an electrical half-wavelength (or multiple thereof) at the test frequency.

73
Iain

[VK2CSW]

Thanks Iain

The ladder line is nominally 450 ohms. So I guess it will work out...
The G5RV should have a half-wave at 14.150MHz.

I guess it will cost nothing but time to try!!!

[ZL1RS]

This exact same technique has been used here several times to find multiples of a half wavelength in 300 ohm TV ribbon at 2M for at least three of my portable EME arrays ... it works like a charm.

If you have difficulty find one half wave on 14.150 try cutting to a multiple number of half waves on a harmonic frequency ... you may find the SWR dip is sharper and get a more accurate result. e.g. 5 half waves at 70.75MHz

73, Bob ZL1RS

[vk1da]

The main feature I observe about the G5RV is that nothing is required to be measured or known. The feedline impedance and length, the presence or absence of a balun, the length of the dipole, all are vague variables that magically make this antenna so popular. There's nothing like a good bit of folklore, vagueness and tradition to make something popular.

I think measuring your feedline impedance is contrary to the spirit of this antenna, which was designed for operation with link coupling from a mismatch-tolerant tube amplifier.

For solid state amplifiers it is a hopeless mismatch on all bands except 20m, not surprisingly because it is actually a 20m antenna being 3 half waves in length so it presents a workable resonance there with a lowish impedance, further it is fed by a half wavelength feedline tuned for 20m. Using an ATU and contriving a 50 ohm match on the other bands may make it look usable but unless the feed system is modified to work better on 80m and 40m, it only works because HF bands are so tolerant of poor antennas. The actual impedance of the half wave line is immaterial on 20m, of course and on other bands it just changes the size of the mismatch.

If you only wanted it for 20m, you could dispense with the half wave feeder and put the coax feed right at the antenna, preferably with a choke balun. That would be a nice 20m antenna.

There are some useful hints on tuning the feedline etc on VK1OD's website. While you are there, take a look at the impedance plots for all bands.

[VK2CSW]

Geez

Forgive me for wanting to experiment with an antenna which seems popular, simple to build and fits in the space I have.

In the twelve or fourteen months I have been pursuing this hobby I have been amazed at the number of times that noobs like me have asked a question only to find that is used as a platform grind an axe that is pretty much unrelated to the question. I guess that those of us who aren't born with a complete knowledge of all things radio, and who have taken steps to learn are an imposition, so on behalf of all noobs everywhere I apologise.

It really makes me wonder why I have spent time and money on this hobby.



Colin

[vk1da]

Hi Colin,

Looks like I didn't put enough smilies into my post to make it clear my comments were not intended as criticism of you or your use of the antenna, but were comments about the antenna itself.

I do recommend you read VK1OD's suggestions for adjusting the half wave feedline to the correct length. his website is http://vk1od.net. You may find like me, that some of the analysis is quite mathematical and much more technical than I need. But the graphs showing losses and impedances on various bands do show how much the impedance of the antenna varies from the ideal.

I find parallel fed dipoles work much better than G5RVs. If you get QST or can borrow it, have a look at the article in the May or June QST about parallel fed dipoles.

But as my wife says, too often we males are asked a question and we give an answer to a different question, usually suggesting what we think is a better solution. Sorry if my response took the wrong tack. Good on you for experimenting though. As Matt says, if it doesn't work, at least you find that out, and if it does you have a win.

73
Andrew

[VK3AUU]

A couple of comments/suggestions.

If you know that the ladder line is nominally 450 ohms, then why are you trying to measure it? Take it as a given that it is correct.

VK1DA is spot on about the design philosophy of the G5RV. The old valve amplifiers with the pii coupler output stages could match almost anything so the comparatively short G5RV became fairly popular as a multiband antenna. If you arm yourself with a Smith Chart and also calculate the impedance (R + J) at the centre of the antenna and at the end of the feedline you may be horrified at some of the numbers on bands other than 20 metres. It is probably easier to follow the VK1OD article, but more instructional to do it yourself.

When all else fails, make an Antenna matching unit with a built in reflected power meter.

Good luck with the experimenting, but also understand the theory of what you are doing.

David