G5RV queries

[K4HXC2]

There has been quite a bit of discussion here and elsewhere about G5RV antennas. Tom, W8JI has done a writeup on it, as well as Owen, VK1OD to name a couple.  It seems to me that while a true and efficient multiband, single wire antenna appears to be the holy grail of antennas, the G5RV comes as close as we've come thus far. All the other types have as many or more problems.

In correspondence with Owen, he mentioned that he thought a true solution for this antenna was a remote antenna tuner mounted at the twin lead coax junction. (MFJ-998RT for example)  which seems to solve the most severe problem with this antenna, that of the huge mismatches that cause so much coax loss and require a tuner anyway. 

In conversation with the tech at MFJ he pointed out I'd need to install a balun between the tuner and G5RV to match them. Thing is that the 4:1 balun they recommended would introduce even bigger mismatches on certain bands and doesn't make sense to me except in a limited number of cases. Wouldn't a 1:1 current balun with coax to twinlead mounting work better? Maybe a Balun Designs 1171. (This would seen to be explained better at http://www.karinya.net/g3txq/tuner_balun/)

I got to thinking about it more and wondered if hooking up the twinlead to the Wire terminal on the remote tuner (normally used for longwires) and ground instead of trying to match to the coax connector with a balun might actually work better? This is what SGC does but theirs won't handle power and MFJs new RT can do legal limit.

What I'm trying to do is have as little insertion loss as possible and reducing the number of baluns and connections is my goal.
OTH, I'd like as little common mode as possible too, and question if they're not possible together.

[W5DXP]

Thing is that the 4:1 balun they recommended would introduce even bigger mismatches on certain bands and doesn't make sense to me except in a limited number of cases. Wouldn't a 1:1 current balun with coax to twinlead mounting work better?

The balun of choice for 80m, 40m, 20m, and 12m is a 1:1 current-choke-balun. It is possible that a 4:1 balun would improve the performance on some of the other four bands but only at the expense of those four bands on which the G5RV is known to be a good performer.

[WB6BYU]

You're not concerned with feedline losses, so a higher SWR itself isn't the issue.

In general, tuners are more efficient when matching higher impedances rather
than lower ones (because lower currents mean lower resistance losses.) 
A 4 : 1 balun might improve the match on some bands, but will lower the
impedance to be matched on all bands.  Besides, matching the impedance
is why you have a tuner in the first place:  let it do its job.  The 1 : 1 balun
will help improve the tuner efficiency, so would be a better choice (even
though the difference may be negligible on some bands.)

[W0BTU]

The G5RV is not the weak-signal-generator that many people assume it is. If you understand its limitations, it's a great antenna.

There are better alternatives, though, depending on your needs. See http://www.w0btu.com/g5rv_antenna.html

[K4HXC2]

I only use the G5RV for 80/60/40 m.  A dual 40/80 trapped dipole or fan won't do 60 well.  The G5RV does those three quite well but my goal is to get the losses and common mode down by moving the tuner from the shack to the antenna.
(I use a T8 for all other bands up to 10) Now if I could come up with a 3 band that will handle full power on 40/80 That might be better than the G5RV but most 2 banders don't have the bandwidth for 80m. Hence the G5RV.

[KE4YOG]

Okay dumb question. I use a double length G5RV with a tuner of course. The coax to the matching section is about 5 feet long. Let me know issues with short run of coax.

[K4HXC2]

Most G5RV builders recommend at least 50' and prefer 75' which they say provides a more manageable SWR at the Radio end. So if there is a problem with 5' I'm guessing it would be greater mismatches, which a good tuner might be able to deal with. The upside for you is lower coax losses..  In my case I have 100' of coax so ideally a perfect match is far better because coax losses can drop my signal quite a bit when not matched. ( I've seen calculators which predict as much as 80% power loss in the coax) So with the tuner at the antenna those losses are almost eliminated. Sort of like getting a free amp. Radio stations almost always put the tuner at the antenna, but they only use one frequency so it's easier to manage. For us, the cost to remotely tune many bands--and run any amount of power greater than 300 watts or so--has been cost prohibitive until just recently, so most tuners were in the shack.

My Dad sort of solved this problem by a two pronged method that was not costly but hard to manage. He had a big cage dipole that he used on a few bands (I think 80/40/20) with a tuner at the base of the ladder-line, he had to go outside to change bands, then a tuner inside for fine tuning within each band. Worked ok with SWRs 3 or under most times, but was a real pita to do band changes as far as I was concerned.


 

[WB6BYU]

Most G5RV builders recommend at least 50' and prefer 75' which they say provides a more manageable SWR at the Radio end. ...

Remember that the G5RV was developed in the days of tube rigs with tunable output stages.
While this could match a reasonable range of impedances, it required a lot of capacitance
to match low impedances on 80m and 160m, which was often the limiting factor in the
matching range.  For example, I remember one of the Johnson rigs was rated to match
from 40 to 600 ohms:  this means it wouldn't match a 2 : 1 SWR at 25 ohms, but had no
problems with a 200 ohm load (SWR = 4 : 1).

A typical G5RV will show an impedance of 15 to 40 ohms at resonance in the 80m band at
the bottom of the matching section.  Even at 25 ohms ( SWR = 2 : 1 ) this could be
difficult to match for some tube rigs.  Adding a quarter wave of coax (about 50', depending
on the velocity factor) acts as a transformer to raise the impedance that the rig
sees (to 100 ohms in this case) and made it easier to match.

That's one reason why G5RV originally recommended the minimum coax length - they
weren't as worried about SWR in those days, just whether they could adjust the rig
to load into it.


But modern tuners are designed for a different purpose than a  pi-network output network
on a tube transmitter.  Most tuners will be reasonably efficient at 20 ohms or so (though
they can get lossy at very low impedances.)  As long as your tuner can match the load
efficiently, using a short length of coax is a good idea as it lowers coax losses.  However,
because losses are typically low on 80m anyway, it doesn't really make much difference:
the difference between 5' and 50' with a 20 ohm load is about 0.2dB.

[K4HXC2]

WB6BYU is correct in that at 80m and 5' you're in decent shape.

Here are a few numbers from VK1OD line loss calculator, just to give you some prospective.

First we'll look at my 100' transmission line using RG-213 at 3.5 Mhz and a SWR of 3 (worst case just for this example)
Losses are predicted to be .347 Db or about 92% efficiency, very close to what Dale said.

@ 40M the efficiency drops to 71% @ 20m 60% and at 10m (the SWR can be quite high in a G5RV on 10m) efficiency can be as bad as 37%. Everything else remains the same but changing the coax to 5' the 10m efficiency changes to 91%.

As you can see, coax length can have a MAJOR impact on how you're getting out with a G5RV. Then it can be made even worse by using a lesser quality or incorrect cable.

Ok, so lets say I have about a Db of loss in my coax for a given frequency. Add another Db for insertion losses for filters, tuners, connections, wattmeter, etc, etc. and now you're talking some accumulated big losses. And as the SWR climbs the losses just get worse.

Worst case, and just for the purposes of what we're trying to get at here, lets say you have a total loss from the radio to the antenna of 6Db. That wipes out any gains you get from a small amp or beam antenna.

So a good cheap way to get a G5RV to become as good as some antennas with gain is to get out as many losses as possible.
Matching the radio/amp to the full length of transmission line is the most important step, in my opinion anyway and just happens to be the cheapest too. Moving the tuner to the antenna could, best case, have me getting out as well as if I spent thousands getting a quality beam antenna (which is VERY costly and quite large for 80m). All with a G5RV!

It's just my humble opinion, but I think it could be argued that the people who complain the most about the antenna, simply didn't take into account how to correctly manage an antenna with some pretty wild characteristics.

If I could, I'd run what some call the ladder line matching section directly to my shack tuner at the balanced antenna connection. However, in my case, that just isn't practical and could introduce RF into the shack. So my approach is to mount a remote tuner and try to limit losses and match everything from the tuner to the radio/amp.