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-   -   Tuning the feedline for my G5RV (http://www.worldwidedx.com/amateur-radio-antennas/52592-tuning-feedline-my-g5rv.html)

AE5RD 04-07-2010 08:05 PM

Tuning the feedline for my G5RV
 
I've about had it. I am very frustrated. I read this website from the internet about tuning my feedline to resonance as a first step to building a G5RV. I cut some radioshack 300 ohm twinlead to 35 ft and hooked one end to a dummyload and the other to my swr meter and then to the radio. I can not for the life of me get the this feedline tuned to a low swr. I am down to 27 ft and the best i can seem to get is 3:1. Has anybody used radioshack twinlead and how many ft is the feedline supposed to be using this junk.

Beetle 04-07-2010 08:54 PM

Quote:

Originally Posted by 161 (Post 191891)
I've about had it. I am very frustrated. I read this website from the internet about tuning my feedline to resonance as a first step to building a G5RV. I cut some radioshack 300 ohm twinlead to 35 ft and hooked one end to a dummyload and the other to my swr meter and then to the radio. I can not for the life of me get the this feedline tuned to a low swr. I am down to 27 ft and the best i can seem to get is 3:1. Has anybody used radioshack twinlead and how many ft is the feedline supposed to be using this junk.

A "G5RV" antenna, by definition, is 102 feet long overall, fed in the center with 34 feet of 600 ohm open line. If you're using regular 300 ohm line, the length should be closer to 28 feet. Then any length of 50-75 ohm coax to the tuner. Any substantial changes to these dimensions yields something, but not a true G5RV.

As a general rule, the only band on which a G5RV will "work well" without a tuner is 20M. That's what Varney, the original G5RV, was looking for. He found that a doublet with those dimensions worked fairly well on the other ham bands at that point in history, but WITH a tuner.

I've never heard of "tuning feedline for resonance" -- and if you go through this pointless drill, a given length of feedline will only be "resonant" at ONE frequency.

What band(s) do you plan to operate with this antenna?

HiDef 04-08-2010 07:12 AM

Can you post a linky to the place you found that?

Dunno why a resonant feedline would be used to feed that antenna.

Resonant feeders were popular before the use of coaxial cable. The feeders themselves were not resonant but their length was manipulated to present a non-reactive load to the transmitter. The combination of doublet and feeder length is trimmed to resonance. Take the feeder length X 1/vf and add one half of the doublet length to figure how the feedpoint behaves. Maybe this is what the site is trying to explain.

BTW, Resonance does not mean 50 ohms load. All it means is the sum total of the reactances is zero. The impedance will act purely resistive. The IEEE definition of resonance INCLUDES maximum current flow which would dictate relatively low impedance feed. This definition differs from previous resonance definitions. Example: a 40 meter resonant doublet will also be resonant on 20 meters because here will be a frequency where the sum total of the reactances is zero. The feedpoint impedance will be in the thousands of ohms in this case so coax feed is not going to work. Some call this situation anti-resonant.

Feedline is also made resonant when it is a different impedance than the source and load. Cheap cable TV 1/2" foamflex makes a great way to get low loss on VHF but it must me trimmed to length for low SWR.

In any case you should have been able to find a low SWR somewhere with what you were doing. Is the dummy load physically large? Can you substitute a 50 ohm non-inductive resistor? A large load like a cantenna or big Bird type might be a problem due to it being unbalanced. Are you testing with an MFJ where you can try a bunch of different frequencies. Is the feedline in a straight line away from large metal objects?

AE5RD 04-08-2010 08:41 AM

Here is the link. Optimising a typical G5RV I am using a small dummy load in a 2"x3" metal can. It is very primitive. The feedline is running out of an aluminum window in a straight line and hooked to the wooden fence on the other side of the yard. There is the minimum amount of objects in proximity of the feedline. I recognize that i need to purchase a MFJ 259b or equivalent. At the time I have limited resources. I took the day off from work today to try to get everything in order. I usually stay on 80, 40, 20 and sometimes 10 but i have the maco for ten meters.

Beetle 04-08-2010 08:55 AM

Connecting a presumed 50-ohm dummy load to a length (any length) of 300-ohm feedline represents, on its face, a 6:1 SWR if the meter is normalized at 50 ohms, as most of them are.

How are you determining "resonance"? As HiDef pointed out, the term simply means that the circuit has zero net reactance, and is purely resistive. Sometimes resonance occurs at an SWR minimum, but often it doesn't.

I see you just posted a link, which I hadn't seen before starting this post. I'll check it as time allows.

SR385 04-08-2010 09:25 AM

Quote:

Originally Posted by HiDef (Post 191991)
Can you post a linky to the place you found that?

Dunno why a resonant feedline would be used to feed that antenna.

A G5RV is three half waves on 20m is why. The balanced feeder is an active electrical part of that design. It is part of the antenna. This is why it is important to have that element hanging straight and free of the ground or metal objects. Ideally a G5RV should be flat topped as well due to the interaction of the balanced feeder and the top two radiating elements.

The VK1OD site is very good for his explanation of constructing a G5RV.

The part that may be fouling up the try here is using the 300Ohm twin-lead and what exactly the velocity factor of it is. If the OP has extra, start with it longer than indicated and see if you get better results. You may be too short where you are now.

Also, don't overlook the requirement of the choke balun at the point where the coax feeder meets the balanced line. It's noted in the instructions, but it's important for a G5RV to operate properly.

W5LZ 04-08-2010 09:30 AM

That parallel feed line is acting an an impedance transformer, converting whatever the impedance of that 102 foot doublet is to something close to 50 ohms that your transmitter likes to see. It isn't 'tuned' in the way 'tuning' is thought of. That's all it does.
If that 'linky' is for determining the original purpose of a 'G5RV', there are probably several around because Varney made it plain that it was a 20 meter antenna, and only -incidentally- could be made to work on other bands. All of the misconceptions with a G5RV antenna are a result of unsubstantiated 'BS' with advertising. You may have a parrot that 'barks', but I don't think I'd try putting a flea-collar on it...
- 'Doc

hookedon6 04-08-2010 10:05 AM

Quote:

Originally Posted by SR385 (Post 191998)
A G5RV is three half waves on 20m is why. The balanced feeder is an active electrical part of that design. It is part of the antenna. This is why it is important to have that element hanging straight and free of the ground or metal objects...


BINGO,.............. we have a winnar:thumbup1:

the feed line in a G5RV does radiate


yikes,..... i just reread what i wrote. i must have 'dain bramage, the balanced feed line has equal and opposite currents on it and does NOT radiate


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