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Link dipole problems


#1

Hi!

I made two years ago a link dipole for six bands which I have used so far successfully. SWRs are typically between 1.1 to 1.4. I used normal 1.5 mm2 electrical installation wiring (five wire). Now I am making new similar antenna but I met problems. With the same wire lengths I am unable to tune the new antenna, SWR’s are more than 3.0 and I do not find the reason! Feeder cable is about 20 meter long RG58.

Can somebody help me, what to do next? I feel me very stupid! Is the length of feeder cable critical or should I change the 1.5 mm2 electrical installation wiring to “real antenna cable” or both? The length of feeder cable of well working link dipole is about 25 meter. Just now I feel me being handless.

73, Saku OH2NOS


#2

Have you checked the DC resistance of the wire - maybe there is a break inside the insulation or a very bad joint somewhere

Colin G8TMV


#3

No expert myself
But have a cuppa tea and a think.
No doubt done something very simple wrong some where along the line.
And stupid has nothing to do with it, this is why we build these things to play with find out where its gone wrong why and sort it out don’t give up

Have heard of these antennas and yet never seen one till looked up on the net.
Very interesting antenna design perhaps might help like to have a go myself one day

Karl


#4

The insulation on the wire can affect the lengths slightly. But for gross errors like these then you should first check for good DC continuity across the feeder and all the sections of the dipole. If that is all OK, then possibly you have excessive RF current on the coax braid and a choke balun at the feed point will help you. (6 turns of the feeder, about 10-15cm diamater, solenoid wound).

Check the continuity first though.


#5

Hi Saku,

Simply use the original length of RG-58 (25m “random length”). Your linked dipole will only work properly with this feedline length (or multiples of it, hi).

Random length feedlines can be used only if each of the dipoles has been aligned originally using a halve-wavelength (or multiples) feedline per band.

73 gl,
Heinz HB9BCB

Some basics can be found here: http://m0kwr.com/zips/Feedline%20Facts.pdf


#6

Thanks guys for hints! In the top of antenna I have screw compression fittings and with 99.9% probability they are ok, but the antenna connector in rig side can be the failure causer. Choke balun is installed according to Andy’s hint. I will check those potential problem causers at home in the evening.


#7

The insertion of the proposed feedline choke does extend your feedline length by about 2.5m. This may improve the values on some bands and worse on the other bands.
Extending the feedline by another 2.5m to the original length of 25m will make you happy.


#8

Hi,
Most antenna problems like this are related to connector problems. I found my BNC connectors to cause problems - my poor assembly methods I guess- so went to simple 2 pin plug and socket. Problem fixed.

If you have to use a fixed length of coax to get the lowest SWR you have a common mode current problem and a current choke at the TX will reduce that. The SWR without common mode currents is independent of coax length other than a small apparent improvement with increased loss for longer lengths.

Feed line dress (whether it comes away at right angles to the antenna etc) can affect whether or not you have common mode current on the outside of the coax braid. Common mode currents are bad because you have increased noise pick up and a “hot” rig.

Fix the connections first, then worry about the common mode currents.

73
Ron
VK3AFW