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Linked Dipole problems!

Im hoping someone can point out the obvious mistake i must be making here!

Im putting an inverted V linked dipole together for my Scotland trip, now, i can get 10m, 12m and 17m to work ok, but for some reason cannot get 20m lower than 3:1 anywhere! Im using my MFJ-259 to test, and even sweeping well above and below the band in cant get it any lower, ive tried more wire, less wire, higher and lower apex, nothing seems to work. Now, i dont have an ATU to take with me, so must get below 2:1 where i intend to operate.

As a rundown on the design, its apex is at 6m, on a fibreglass fishing pole, and the tie off points for the dipole ends are at 8m radius. At 20m this brings the ends of the dipole to about 1.5m above the ground, which i would have thought to be ok. The dipole is made using loudspeaker wire and automotive bullet crimps.

Im sure im doing something obvious wrong, but i cant work it out.

Would someone else who uses one of these on the summits let me know what dimensions they use? Do i need to use a greater radius for the ends of the dipole? I cant easily test that, as i dont have the garden space!

I havent even tried to make it work on 40m yet, which is the main band i intend to use!

All help much appreicated!

In reply to G7MRV:
See http://www.flickr.com/photos/john_clifford/535686398/
as a start, you have 12m & 17m sorted so just need to slot in 15m and possibly 30m
HTH Roger G4OWG

In reply to G7MRV:

Hi Martin,

Assuming that your dimensions are correct (pretty close to 4.77M per leg for 14MHz based on John’s diagram) I would check for DC continuity through your links when they are connected & that there is nothing physically close to your inverted V that may be resonant on 14MHz. Ideally, any testing should be carried out in the clear, away from other objects, especially metal ones.

Good luck,

73,

Mark G0VOF

In reply to G7MRV:

I found info on the web (sorry can’t remember where) that says the lengths need to be 3% shorter than for a horizontal dipole for an inverted V where the droop angle is 30 degrees (i.e. the arms are 30 deg from horizontal).

When building mine I also found that if the loops of wire that bridge the insulators are too long then you get high swr, this can be minimised by making sure that the loops are as short as possible and also by using something to hold the unconnected end of the last one back close to it’s element rather than just letting it dangle (when I presume it acts as a capacity hat).

Colin G8TMV

In reply to G7MRV:

Where is the balun?

Andy
MM0FMF

In reply to G0VOF:
Ive checked the links, cant find anything amiss

Im having to test in the garden, and of course that means my doublet is only a few meters from the feedpoint of the inv.V. I might see if i can get out to the local football fields and test it there.

Failing that, im going to cut a 40m only dipole as well, this was intended as a backup! but might end up being the main antenna.

In reply to MM0FMF:

Replying to myself…

http://www.sotawatch.org/reflector.php?topic=5950#

Andy
MM0FMF

In reply to G7MRV:

Just a thought, check that both sides of the 20 metre section are the same length, if you have made a cutting error and one side is longer than the other, then even at resonance there will be a mismatch to the feeder.

73

Brian G8ADD

In reply to G7MRV:

Ha, I think im wayyyy off! Just measured a leg, and 4.77m comes to the middle of the 17m section on mine! Not at all sure how thats come about!

Odd that i could get the first few sections to work, but i’ll remeasure and rebuild (probably quicker to rebuild to Johns dimensions!)

In reply to G7MRV:

I’ll ask again as you’ve not replied.

Where is the balun?

Andy
MM0FMF

In reply to G7MRV:

Measured the legs with the multimeter, no continuity on one leg beyond the 17m link. Pulled firmly and the crimp came off.

In reply to MM0FMF:

No Balun. I doubt a balun would have any advantage, and is just more weight.

Now resonant a little high, 15.4MHz - one of the crimps to the 17m section had cut through the wire! Redone and its now 1:1 but a meg high, whereas before the closest point was 3:1 at 17MHz!

I knew crimps were a bad idea. Going to look on ebay now see if i can get a pack of slide switches cheap

I think best to scrap this version, and wait for the switches.

In reply to G7MRV:

Going to look on ebay now see if i can get a pack of slide switches cheap<

Cheap slide switches at the high voltage end of a dipole combined with British weather seems like a recipe for disaster to me !!!

4mm wander plugs and sockets might be a better idea or even some short links with crock clips on the ends.

73 de Ken G3XQE

In reply to G7MRV:

I doubt a balun would have any advantage

Really?

Is that “doubt” based on a thorough understanding of how currents flow on the feeder to a dipole when fed with coax or because you can’t be bothered with one?

:slight_smile:

Andy
MM0FMF

In reply to G3XQE:
Indeed, you could be right. However, ive ordered some now, so will try them out. They’ll get a coat of something to keep the weather out. I’ll only be running 10w max so hopefully they will survive, and the isolation be enough to make them behave.

The 4mm plugs and sockets are much dearer, and i dont have enough croc clips (besides crocs and my fingers dont seem to get on!)

Added about 67cm more than Johns design but got the 20m section working. Going to add the 40m now, then will rebuild later with the measured dimensions

In reply to MM0FMF:

Its based on being a time served communications engineer, public safety and broadcast.

Yes, there could well be some common-mode current due to the unbalanced feed, but its effect is likely to be minimal, and other than pattern degradation, its likely to be of little concern. Im not likely to cause much RFI on a mountain top!

In reply to MM0FMF:

Hmm, for some reason my posts keep getting posted twice, my appologies.

Well, it turns out all my troubles have been down to the crimps, i think (wont know for certain until i section one of the used ones) that the crimping tool is crimping the back edge too tight, can its cutting the wire where the sleeve starts. The trick i think is to either crimp well clear of the sleeve, or to over crimp and include some of the sleeve, but its hard to get it into these crimps.

Another one had failed (annoyingly the one i’d just replaced!!!) and it dawned on me while trying to get the 40m section away from 5MHz! After a few tries and the resonant point was still 2:1 and no better around 5.7MHz, i went back and checked the crimp continuity. After fixing that crimp, easily resonant at 7.12MHz

Thanks guys for the help. Its annoying how often such simple things get overlooked, and throw you off the scent

In reply to G7MRV:

I used soldered radio control bullet connectors in my linked dipole. Very compact and neat. There are plenty of ebay sellers. eg,

http://www.ebay.co.uk/itm/5-X-2-0mm-2mm-Gold-Bullet-Connector-for-RC-battery-/300471593163?pt=UK_ToysGames_RadioControlled_JN&hash=item45f580a8cb

Also in 4mm size.

Thanks

Richard
G0IBE

In reply to G0IBE:

I like the look of those! The lack of insulation appeals, bizarrely, because i’ll be able to see if they have made correctly!

How do you find them for connecting/disconnecting in the cold, with gloves etc? and what sort of power level do you run with them? The 4mm types might be in the cold

For the sake of a couple of quid i could try those and the switches.

Edit to add: ‘Bought’ the bullet! Ordered a couple packs of the 4mm types. If they arrive from 'ong kong before my holiday i’ll try them out.

Just one final check on the dipole, to see what it looks like on 15m.

In reply to G7MRV:

Hi Martin

I use a linked dipole (Inv Vee) for 20, 30, 40 & 60m.
I use Anderson Powerpoles as a connector between each section. I also had a bit of trouble with the initial tuning. Firstly, because I kept changing the angle of the dipole every time I added another section - thereby changing the feedpoint impedance and secondly because I was trying to tune it in a confined space. My SWR readings are all very low on each of the 4 bands, subject to the terrain over which the antenna is erected. I don’t use a balun nor a tuner because I don’t want to carry them up a hill!!

Regards

Dave