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 Dipole Shortening. 
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Post Dipole Shortening.
Just a quickie.

We all know the old trick of shortening a dipole by putting some ladder line in each leg at 90 degrees, like a trombone, to make up the length.

Question. Does it have to be 'big' open wire ladder, or will some 300 ohm 'window' feeder nicked from an old g5rv do??

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Chris, 'oop North.


Fri Apr 13, 2012 4:26 pm
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 Dipole Shortening. 
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Post Re: Dipole Shortening.
I've seen a couple of designs about that use 300 ohm.

One was in an article in PW some time back called the "Quipp" Antenna.

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Fri Apr 13, 2012 4:54 pm
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 Dipole Shortening. 
Silent Key

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Post Re: Dipole Shortening.
It's a bit misleading to think of it as "making up the length"; what you are actually doing is inserting a loading inductance formed by a length of short circuit transmission line.

You can use any transmission line you like, including coax, but ladderline or open-wire line will have lower losses.

The length you need for any particular inductance is determined by the characteristic impedance of the line and its velocity factor. For example if you needed a 10uH inductance to load a short 20m antenna, you'd need:

10ft of 600 Ohm open-wire line and the inductor Q would be 380
11.2ft of 450 Ohm line and the inductor Q would be 260
11.1ft of 300 Ohm tubular and the inductor Q would be 54
11.1ft of RG213 and the inductor Q would be 5.6

As you can see from the varying lengths, it has nothing to do with "making up the missing part".

Hope that helps,
Steve G3TXQ


Fri Apr 13, 2012 6:36 pm
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 Dipole Shortening. 
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Post Re: Dipole Shortening.
G3TXQ wrote:

Hope that helps,
Steve G3TXQ


Thanks boss, I'll tell you what I'm up to, and you can tell me if you spot any 'stoppers'.

I'm trying to 'design' an antenna for my new small QTH. To cover the bands (80 - 40 - 30) that won't go in the loft.

Start off with a half-sized G5RV. Trim it so that the legs are both about 20 feet long and the twinlead is about 10 feet long.

Couple-up a 1:1 balun and coax, then hoist the middle up a fibregass pole about 17 - 18 feet, and tie-off the ends to the garden fence about 6 feet up.

Nibble the feeder to tune it to about 8.5 megs and take up the VF of the twinlead.

OK, what you should now be looking at is a reasonable, but not wonderful, NVIS antenna for 40m and 30m.

Next.
Separate the balun from the twinlead and insert a DPDT knife switch in a plastic butty box. When the switch is down it connects the balun to the twinlead as before. When the switch is up it disconnects the balun and shorts the twinlead together at the bottom.

Now, if i then use a 6 foot flying lead and crocodile clip to feed one end of the dipole against earth, then I will have a 66 foot length of wire (if you include the flying lead) with 20 feet of loading in the middle, which should load-up (ish) after dark on 80m. Sounds too simple ... I must be missing something :-)

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Fri Apr 13, 2012 6:56 pm
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 Dipole Shortening. 
Silent Key

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Post Re: Dipole Shortening.
Chris,

This is how it would work on the three bands:

30m: the impedance at the end of the ladderline would be a reasonable match to the coax - SWR about 3:1 - so it should work pretty well.

40m: the dipole is only 60% the length of a half-wave, so its impedance is very low. The SWR on the coax will be over 250:1, so you can expect lots of loss.

80m: The ladderline doesn't provide any loading at all; instead, what you have created is a 16ft vertical (the ladderline + the flying lead) with top loading (the two 20ft legs). It can probably be made to work, but would require an excellent low-loss ground system to keep the losses low because the radiation resistance of such a short vertical is about 3 Ohms. If you feed it directly with coax the feedline losses would also be high, so you really need a loading coil at the base of the flying lead.

73,
Steve G3TXQ


Fri Apr 13, 2012 8:25 pm
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 Dipole Shortening. 
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Post Re: Dipole Shortening.
I don't really get that to be honest. I understand how it might be 'a funny one' to your modelling program, but.

If the twin feeder was only a few feet longer or shorter it would be a near 50 ohm match on one or other of the bands, that's how a G5RV works. Somewhere along the twinlead there is a current lobe and a Low-Z feedpoint.

On 80 it is just a mangled inverted 'L' with a big sudden sag in the middle. The current flowing through the shorted feeder will be nearly equal and opposite, and the current in the arms (especially the one nearest the feedpoint) and the flying lead will do the radiating.

I'll just build it and see how it goes.

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Fri Apr 13, 2012 9:48 pm
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 Dipole Shortening. 
Silent Key

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Post Re: Dipole Shortening.
G6CSL wrote:
If the twin feeder was only a few feet longer or shorter it would be a near 50 ohm match on one or other of the bands, that's how a G5RV works. Somewhere along the twinlead there is a current lobe and a Low-Z feedpoint.


On 40m the dipole is far too short and the ladderline is too short to produce an impedance anywhere near 50 Ohms. A Half-Size G5RV comprises a dipole 51ft long and a ladderline 17ft long; that combination delivers a VSWR(50) under 3:1 on 40m. Your dimensions are simply too short for 40m, although they are pretty good for 30m.

G6CSL wrote:
The current flowing through the shorted feeder will be nearly equal and opposite, and the current in the arms (especially the one nearest the feedpoint) and the flying lead will do the radiating.


Apologies - I misunderstood how you were feeding it on 80m - I thought you were feeding it between ground and the shorted junction of the ladderline.

Feeding it at the end the way you propose, the ladderline does indeed add some loading to the dipole - equivalent to about 4uH; so what you have is an end-fed wire 46ft long with a 4uH loading coil in it 20ft from one end. You can't include another 20ft in the length because - as you said - it's carrying balanced currents and can't radiate; all it can do is behave like a loading inductor. Unfortunately the 4uH it represents is far too small to change the feedpoint impedance much - you'll still have an SWR of over 100:1 if you feed it directly with coax. You need an inductance of around 30uH to bring it to resonance, and then the SWR on the coax would be down to 10:1.

Why not arrange for your switch to insert some inductance on 80m instead of simply shorting the ladderline? A 15uH inductor across the ladderline ends would be equivalent to 30uH up at the dipole.

73,
Steve G3TXQ


Fri Apr 13, 2012 10:20 pm
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 Dipole Shortening. 
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Post Re: Dipole Shortening.
"Why not arrange for your switch to insert some inductance on 80m instead of simply shorting the ladderline? A 15uH inductor across the ladderline ends would be equivalent to 30uH up at the dipole".

That's crafty idea. I could put a horizontal coil round the back of the mast in place of the shorting link in the box. Anything to pull more of the current lobe up into the dipole on 80.

I didn't realise that the shorted feeder would have so little effect. I thought it would act a bit like the 'trombones' between sections of a colinear, where they take the voltage spike at the end of one section and transform it into a current feed for the next section above.

I'll try the twinlead length by trial and error, and likewise make the dipole arms as long as possible when I actually put it in the back garden. I only set the coax/balun/switch/feeder junction at that height so that you would be able to reach the switch when it went dark :-)

So, if I can get away with longer arms and/or more twinlead, and a bit more mast height without needing a ladder to get to the box, then all the better.

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Fri Apr 13, 2012 10:52 pm
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 Dipole Shortening. 
Silent Key

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Post Re: Dipole Shortening.
G6CSL wrote:
I didn't realise that the shorted feeder would have so little effect. I thought it would act a bit like the 'trombones' between sections of a colinear, where they take the voltage spike at the end of one section and transform it into a current feed for the next section above.


On 80m the 10ft of feedline equates to a tiny 0.037λ and there's not much impedance transformation from one end to the other. With a short circuit at one end, you get pretty much a short circuit at the dipole. You'd need it to be about 45ft to give you the required 30uH at the dipole.

If you arrange to switch in some inductance, you can "tweak" it for minimum SWR at the feedpoint. In theory Rrad will be about 5 Ohms; if we assume ground losses of 10 - 15 Ohms you should see an SWR between 2.5:1 and 3.5:1.

73,
Steve G3TXQ


Sat Apr 14, 2012 11:31 am
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 Dipole Shortening. 
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Post Re: Dipole Shortening.
Excellent Steve, and a big big thanks for all the number-crunching.

I'll have to look up what a 15 Mu H coil looks like. I'm assuming it's not the size of a dustbin?

OK on the SWR on 80. Sounds like another job for the old 213 :-)

I'll just try it for 40 and 30 first, then add the other bits nearer to when the clocks go back. It would be nice to find a mid-point where the SWRs on 40 and 30 were still manageable, but otherwise I'll just leave it on 40 and think of something devious in the loft for 30.

It might mean running the last few feet of the dipole ends along the top of the garden fence, but at least it will keep the cats away.

Best Wishes, have a nice weekend.
Chris

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Sat Apr 14, 2012 11:47 am
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Post Re: Dipole Shortening.
OK, I'll just recap for anybody who is interested, to save them reading the whole thread.

What we are doing is using a commercial half-sized G5RV for high-angle inter-G work from a very small garden, on both 40m by day and 80m by night.

Instead of strapping the feeders together and using the antenna as a 'T' on 80 (which produces low-angle radiation) we are going to instead disconnect the 40m feeder altogether, and drive the antenna against a good earth (via an ATU) from a second, totally separate feedpoint at one end of the dipole, so that it is instead 'a 1/4 wave wire'.

This is first done by setting the standard half-G5RV up as an inverted V, for 40m only, fed using a 1:1 balun and co-ax in the usual way.

Next, a 'DPDT knife switch' (see google) in a plastic box is connected between the balun and the twin feeder. This is wired so that when the switch is 'down' it connects the balun to the twin feeder in the usual way, but when the switch is 'up' it disconnects the balun from the twin feeder, and instead shorts the bottoms of the twin feeder together via a 15 microhenry coil positioned around the back of the mast. The twin feeder will then need to be trimmed back to resonance on 40m.

A flying lead with a crocodile clip is then used to connect the second feedpoint to one end of the dipole. The coil is then trimmed to bring the 80m SWR within the range of your ATU.

So, when it goes dark, all you need to do is pop out in the garden, throw the switch and connect the flying lead.

I'll be trying this out after I move to my new QTH shortly, but if anybody else tries it before then, please let us know any problems that you find.

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Sat Apr 14, 2012 1:57 pm
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