The G5RV - or a doublet antenna of no particular significance

[Vin Robinson]

Hi to all the Group Members.

This is my 1^st contribution and I see the hot topic is the “G5RV”

Seeing all the useful comments and some confusion, I hope what follows helps.

 

I think I need to find a slot for my “The Optimised Doublet Antenna” talk at the Club which I’ve given 3 times in the last 20 or so years. Lets try and set down a few facts about doublet antennas, which the G5RV is merely one of millions of options and has little relevance to the 10 HF Bands available to us. What follows applies to all HF doublets and I'm sure many of you know a lot of this already.

 

1. The  doublet is a non-resonant antenna  that is intended to be used over very wide range of frequencies. (Yes, I know it’s dipole length will give it 1 resonant frequency). This gives it a huge advantage over most other antennas except perhaps long wires and variants such as Zepps and extended Zepps and these are much more noisy.
2. It must be fed symmetrically in the middle with balanced transmission line eg 600 ohm open wire feeder or the best 450 or 300ohm slotted line.
3. At any frequency of operation it will almost always have a very high SWR on the feeder, easily 20, 30 or 40:1, it doesn’t matter,  ie, there are always very significant standing waves on the feeder and thus it will present a very complex impedance at the ATU end of the feeder resulting from the very highly reactive load seen by the ATU. All these depend on the frequency of operation, length of antenna, height above ground, geometry (eg inverted V etc), nearby objects and very critically, the length of the feeder.
4. The very high SWR does not matter at all because of the very very very low loss of open wire line, much much lower than RG213 for example. This means that the additional “SWR” losses are insignificant, look at the critical graphs of matched and unmatched lines losses in all the standard handbook and antenna texts to understand the theory, it’s not complicated.
5. There is no amateur band where a 5RV looks like a 50ohm load.
6. No two 5RV’s anywhere in the world give the same “load” at a specific frequency.  And  no ATU will ever cover all solutions for matching into a 5RV in a particular situation for all frequencies, therefore, it’s important to own more than one ATU and of different designs. However, most ATUs will match most of the frequencies. Adding or subtracting lengths of feeder is the easiest way to get a match where the ATU needs some help.
7. A very important limitation is that the lowest frequency any doublet can be used on efficiently is the one in which the length is about 80% of the length of the equivalent dipole at that frequency (which is why a 5RV is useless at 160M but ok on 80m)
8. At the other end end of the range, a very long doublet ie 2 or more wave lengths long will generate very specific lobes and very deep nulls which may or may not be useful eg a full size 5RV is poor on 10m (which is why I use a 51ft top doublet for 18MHz to 28MHz, more on this later).
9. A doublet should be matched and resonated with a Balanced ATU, with the feeders going right up to the ATU terminals . The Z Match is excellent for this and it has only 2 controls. If however, you have only a “coax to coax” ATU or similar with a balun internally, as most people have, then read on below.
10. Baluns  --  I agree with Don, G3XTT  and Dave, M0DHO, that generally these are a bad idea as they are meant only for transforming resistive loads and not complex impedances. Ie reactive loads. However, if that’s all you’ve got and the ATU will tune up and the balun does not get hot (very bad news if it does), then fine, it probably doesn’t matter for that particular band.  However, most commercial 5RVs are sold with a length of coax extending the “magic” 34ft length of balanced line to the ATU which you can plug straight into the ATU coax socket. A few have a balun at the coax – balanced line junction. A short bit of coax, say under 5m  probably doesn’t matter but a long length is really bad news as the frequency increases. The very high SWR losses on coax, especially RG58/UR43  are simply too high. The best to extend the balanced line with more balanced line. You can drop down to a short length of 75ohm twin or 300ohm ribbon to go through the wall of  the house.
11. You can try different ratio baluns the improve the match and a different one might not get hot but you don’t have to use a balun, you can simply go from coax to balanced line in a passive junction. The system won’t be properly balanced but it probably won’t matter in many cases and will give you another range of options to get a match and eliminate an item which may be getting hot at one or more bands. The important thing to remember is that once the ATU has resonated and matched into  the Antenna/feeder system, the radio thinks it’s working into a 50ohm resistor and will transfer all its power to the antenna. The only losses will be the ATU losses which are generally about 5%-8% and the additional SWR lines losses. ( at 80m, a matched balanced line, 10m long  has line loss of only 0.01db.  With a 20:1 SWR on the line,  the additional  losses are only another 0.015db, totally insignificant, but if there also a long length of coax extending the line, then the losses are very much higher, many dbs, particularly on 10M)
12. At frequencies higher than the ½ wave dipole length equivalent frequency, the antenna begins to develop very good broadside gain. For example, a 51ft top doublet is roughly a double extended Zepp (ei 2 x 5/8^th waves in phase) on 18MHz. At ½ wave about ground this generates a low angle, 30deg elevation lobe off each side of the wire giving about 7db gain over a dipole.  There are many other models which can be quoted from the antenna modeling programs which give high broadside gain and the expense of end fire gain.. Beyond 3 ax ½ waves in phase the advantages diminish as in the 1M band for a full sized OR case quoted in 8 above
13. So, you’ve built a doublet which is fed by 450/600 ohm balanced line and tuned with a I Match or other balanced ATTU. This is a superb antenna system because:-

 

1. It’s multi frequency.
2. It’s quiet compared with end fed wire antennas and unbalanced dipoles because noise is cancelled out if a good balance is preserved.
3. The antenna can be bent around, dog legged, inverted I'd and generally crammed in to small spaces within reason and the ATTU simply deals with the changed resonance at the expense of a loss of balance.
4. Good broad side gain can be achieved for several amateur bands if the length is carefully chosen, use a modeling program.
5. It’s cheap, all you need is a roll of suitable wire and some spacers. It’s also practical and can be adapted for most gardens.

 

Finally, I worked 3B7C  on 3  HF bands using my 51ft top doublet and 100W SSB without the slightest difficulty, ie almost 1^st call,  so  once you’ve built the right one and matched and resonated it properly, they are brilliant and don’t let anyone tell you they aren’t!!!

 

Vin, G4JTR.

Chairman RADARC.

[James Hopper]

Hi All,

         I vowed not to get involved in a G5RV discussion, but could no longer resist. A very good description from Vin, but I would like to add some clarification on the subject of baluns.

 

If you have a proper balanced ATU - a Z-match or similar, (as Vin says, it's the ideal way to drive a G5RV), then you can ignore the following, but if you're using the more common 'T-match' style of ATU (even if it has a 'balanced' output), then the following may be of interest.

 

There are generally 2 type of balun in common use in antenna systems:-

 

        a.) 1:4 voltage baluns. This is the type included in most T-match ATU's, and is intended to match a balanced RESISTIVE load of 200-300 ohms (driven via ladder line) to a nominal 50-75 ohm unbalance (coax) line. It generally a BAD IDEA to use this on a G5RV, which is generally a very REACTIVE load of variable impedance. The losses in the balun can be high, which can cause it to overheat and fail at non-qrp levels.

 

        b.) Current baluns, generally 1:1 (unity) ratio. These are also known as 'braid breaker', choke, or isolating baluns. Their function is to 'choke off' any unbalanced currents in the feeder, to ensure all the power is radiated from the antenna, keeping it out of the shack wiring. These baluns can take the form of several turns of coax through a ferrite core, coax wound around ferrite rod(s), or just several turns of coax either wound in a loop, or around a lemonade bottle etc. Although it may seem strange conceptually, if you run ladder line all the way to the ATU, you can put a 'braid breaker' on the ladder line, though you need be careful with the layout of the balun, so as not to violate the rules of the ladder line. A short piece of coax wound around a ferrite is usually easier! In my opinion, a current balun, whilst not essential, is usually an asset to a G5RV installation.

 

On some of the better ATU's, the manual may contain a description of how to re-wire the internal voltage balun as a current balun. There's plenty of info about using these types of balun with a G5RV on the 'net.

 

With a voltage balun, all the RF power must pass through the core, hence it's lossy if used on a reactive load (it can be quite lossy even with a resistive load!). In the case of a current balun, it's only effectively 'in line' with the ground current, and it's trying to stop this by presenting a high impedance, so the losses (if designed correctly) will be very low.

 

As the antenna layout is rarely symmetrical to start with, there will always be unwanted RF currents in the shack/house wiring, but a good choke balun can go a long way to reduce this. It's particularly handy if you're operating in an upstairs shack with PME, where an external RF ground would be ineffective due to its length, and undesirable for safety reasons. As antennas are by nature reciprocal, a good choke balun will also reduce QRM picked up from the shack/house wiring in rx mode.

 

If you strap the feeders to run the G5RV on 160M against an earth counterpoise, you should generally remove the balun.

 

73,

     James   G8XML

[Andy Cunningham]

James Hopper wrote on 16/05/2009 23:19 :

        a.) 1:4 voltage baluns. [...]

 

        b.) Current baluns, generally 1:1 (unity) ratio. These are also known as 'braid breaker', choke, or isolating baluns.

I'm still not sure I properly understand the difference between the two - and also the terms "voltage fed" and "current fed"....   Would anyone like to enlighten me?

Andy
M0HAK

[Denis Pibworth]

Hi Andy (and all)

This is an interesting one..

All system are voltage and current fed otherwise no
power would go into them ;-)

If a system is high impedance then is referred to as
'voltage fed' - if 'low impedance' then current fed...

I don't know if there are any rules of thumb as to
above or below what impedance these expressions
are used...does anyone else know ?

Regards

Denis

--
Denis Pibworth G4KWT
20 Marathon Close
Woodley
Berkshire
RG5 4UN
0118 969 8526 (Home)
01256 387950 (Work - DDI)
07970 269308 (Mobile - Orange)
E-Mail : de...@pibworth.org.uk

[Eric Curling]

Hi All,

Ignoring the advertising blurb, this is a really good site with lots of very valid data that may lay most of the Balun questions to rest.

http://www.dxengineering.com/TechArticles.asp?ID={3E5220F7-2D0F-45B5-85F7-3B654F804C4F}

73's  Eric (M0LUV)