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Easy balun match for my dipole


#1

While I’ve not ruled out making a portable beam antenna, I’m having to stick with my 1/2 wave telescopic on my HT and my homemade 1/2 wave open dipole on the end of 2 metres of RG58 (both vertically polarised, I’m FM only).

While I’ve been able to do quite well at activating I’m aware that my simple dipole has no balun, so my SWR is going to be around 1.5:1, assuming I got the dimensions right.

How much of a problem is that for a 5W HT (Kenwood F7E)? Is it small enough to not worry about? The radio seems to not mind, I’ve had no issues. But it’s wasted power.

Assuming I should fix it, I looked at hairpin matches for beam antennas. Are they suitable for dipoles too? If not is there a simple alternative? I’ve not many tools to hand sadly, but can maybe borrow some workspace at work.


#2

I made a J Pole - It has a bit more gain and by carefully tuning it seems to have low SWR - there are loads of designs but these instructions worked for me! https://www.essexham.co.uk/slim-jim


#3

Hi,

Your dipole has about the right impedance so your power is almost all being radiated, the only possible issue would be current down the outer surface of the coaxial cable which may detract from the radiation pattern of the half wave. To combat that, make a choke using the feedline. Wind the coax into a choke of 7-8 turns at a 25mm diameter. Or use the outer of the coax as part of the antenna as in the coaxial dipole design. There are some good construction articles describing a vertical half wave antenna called the flower pot vertical, on the website of vk1nam.wordpress.com, and on the designer’s website (vk2zoi). Even if you dont copy the design exactly, it would give you some ideas for chokes made from coax. In that application the coil forms a resonant circuit withe the capacitance between the windings, but if all you need is to ensure there is no rf current on the outer surface of the feedline, it would work for you too.

73 Andrew VK1DA/VK2UH


#4

Don’t worry about a 1.5:1 SWR, Mark - especially at low power, anything under 2:1 is fine. If you can find someone with an antenna analyser, you’ll be able to mark those telescopic whips where they should be set, to be exactly at resonance on say 145.500 - that will then be a 1:1 SWR.

If you want to improve your antenna though, the suggestion of building a J-Pole antenna is a good one as you already have some way to mount the antenna.

Or another option - when you don’t want to take the mast with you is the cheap RH770 antenna bought with the appropriate connector so that it it fits directly to the HT. It might not be quite as good as your proper dipole but it’s close and more convenient.

If you do decide to look at a beam - take a look at a Moxon - they are physically more solid than a 2 element Yagi and performance wise are between a 2 and a 3 element yagi.

73 Ed.


#5

Thanks everyone.

I do intend to build that Slim Jim. That guide looks excellent and I found it previously. I should be able to mount it off the top of my walking poles - I’m reaaaaonably sure there isn’t much metal in them…

Is the RH770 likely to be any better than my current telescopic, which I have the right lengths marked on? It doesn’t have sections, just one long telescope, but I can extend it very accurately.

Re the dipole: what about clip-on ferrites? Is that likely to help? I’m sure the choke is easy to make, will need to get a longer bit of coax to accommodate it though.

Coax dipole looks like it may be annoying to make. Is folding the braid back really that easy? Looks fiddly.


#6

Incidentally, is the gain from a dipole and slim jim or j-pole not the same? They’re both half wave radiators?

I did read the take off angles are different though. Maybe that’s the advantage.


#7

Feeding the Jpole is easier because the feeder is from one end whereas with a vertical dipole the feeder needs to be supported for 1/4wave at 90degs to the elements. It’s easy to obsess over antenna performance and SWR etc. What is more important for SOTA is ease of deployment, reliability and that the match is good enough.

Handhelds are designed to work into terrible antennas as you move the handy about and the environment changes about the antenna. Anything under 2:1 is fine and under 1.5:1 is great.


#8

Yeah, I’ve finally put a proper 90 deg feed line on the dipole and it works great, but sometimes I worry about attaching it to the walking poles. Perpendicular, like.

The JPole or Slim jim would be much easier as the walking poles are wider at the top at the handle so there’d be some free distance away from the poles.


#9

Gain and pattern for a Slim Jim, J pole, and vertical dipole should be essentially the same.

wunder


#10

Hi Mark,

No need to fold the braid back. Look at this design from Andrew VK1AD
Portable 2m 144 MHz Coaxial Dipole Antenna
I made one and it works very well.

73, Mirko


#11

I suppose for sheer portability and ease of mounting the dipole should be ditched. Especially if I can get an equivalent gain/pattern from a vertical-style antenna, hung or otherwise.

I’m very proud of that dipole though, obviously :slight_smile:

I’ll check that out, thanks Mirko.