Questions re the J-Pole antenna.

My local Cadets have a need for a VHF vertical aerial that is robust enough to mount and dismount in quick time and is not encumbered with radials and stuff.
The Centre frequency is 151MHz, +/- 3MHz to cover the band.
Bearing this in mind, some wider-diameter tube is indicated (lower Q) on a J-pole.

The main material is Aluminium tube (32mm diameter) with the 'other' part of the Q-match being of 20mm tube.
Calculations indicate a "twin line" of about 540 ohms.
Good ohmic contact is made with a quarter-inch bolt through both tubes.

The question is this: Should the quarter-wave section be shortened, as in "end effect" (like the radiator), and if so by roughly how much, from the calculated length of 18 inches.
 
I propose feeding it with RG8 coax and a balun.

Thanks.

Questions re the J-Pole antenna.

My local Cadets have a need for a VHF vertical aerial that is robust enough to mount and dismount in quick time and is not encumbered with radials and stuff.
The Centre frequency is 151MHz, +/- 3MHz to cover the band.
Bearing this in mind, some wider-diameter tube is indicated (lower Q) on a J-pole.

The main material is Aluminium tube (32mm diameter) with the 'other' part of the Q-match being of 20mm tube.
Calculations indicate a "twin line" of about 540 ohms.
Good ohmic contact is made with a quarter-inch bolt through both tubes.

The question is this: Should the quarter-wave section be shortened, as in "end effect" (like the radiator), and if so by roughly how much, from the calculated length of 18 inches.
 
I propose feeding it with RG8 coax and a balun.

Thanks.

I've just got a demo copy of EZNEC and I cannot seem to get even  a simple dipole up.
Can you send me the necessary file please?

dave@m0uxb dot wanadoo.co.uk

Have you also tried your SWR meter BOTH ways with a dummy load? That way, you get to see that both detectors are identical.

You can feed a half-wave dipole two ways, by & large. Centre-fed (HF, for example), or end-fed (VHF, typically).

The trick is how to match the very high impedance of the end-fed dipole (zero current, High voltage = high impedance).

A quarter-wave stub has zero impedance at one end (the closed end) and very high (infinity?) at the open end; but this only works where the stub is TUNED to the frequency required.
So, a tuned stub and a resonant half-wave dipole will work quite well (opinions often differ about just how well). It's just a question of finding the right tapping point on the stub.

Incidentally, the J-pole is best fed with a balanced feed. This reduces possibilities of changes to the radiation pattern as well as 'screen currents' and a tricky VSWR.

Suggest you have a look at HAMUNIVERSE.com.

Good Luck
Dave

WB6BYU de G8UYZ

Thanks, Dale.

Construction is coming on slowly. The "lower half" of the dipole is actually outside the tube. 4 strips of flat copper strip soldered to a small piece of large copper water pipe.

I've made some 31mm spacers (from Perspex) to keep things roughly central in the 32mm bore or the plastic pipe.

The 'top' element is an 8mm (27/64th) brass tube. Spacers mounted thereon carry four 2.5mm wires soldered at the top and bottom of the brass tube, this giving an effective larger diameter yet lighter weight. I don't have good enough drills, etc., to make more holes.

Decisions about chokes can be taken later. I hope to find some suitable Ferrite cores (30mm outside, 10.5mm bore). A coax choke is difficult with RG213 cable, unless there's some secret way of joining them up.
One thing though; should a sleeve choke be an Electrical quarter-wave, or a physical one?


If you'll be kind enough to drop me a line directly, I'll tell you more and how it goes on.

Many Thanks to you and all the contributors.

dave@m0uxb.wanadoo.co.uk

Would this 'dipole' be best fed with a balun from up the inside of the bottom element ?

Most analogue meters with an EHT range feature a load of high-voltage, high value resistances built into the probe.
Personally, I'd check with a low-voltage supply to see what that range really does need for FSD first.
After all, it's bit risky having 5kV where you don't actually need it.
And yes, I think I'd use Automobile EHT cable for the job, and lots of heat-shrink insulation.

Good Luck

35" TOTAL length ?
Split in the middle ?

WB6BYU
MAny thanks, Dale. To answer some of your questions:-

1) I'd prefer no radials at all unless they are folded downwards. The youngsters concerned are no respectors of any kit when they come to pack up and depart the site (they can and have lost a 6ft mast pole last time). The rule, by & large, is that it can be broken, bent or battered, it will be.
As if that's not enough, there is a moderately serious risk of some idiot doing some damage to either another cadet or some other piece of kit, like a tent.
So even rounded, blunted, cork-strewn ground plane elements are not high on the list of ideals (unless I can make a transit box for the whole thing). If the aerial can be successfully fitted into a chunk of white waste-water pipe, it will be far better.

2) Yes, it's for 50 ohm coax feeder (usually RG213 or URM67). Desired SWR is better than 1.5:1 one the channels of use (the radios we have will fold back to very low power on anything more). A 75 ohm aerial, presenting about 1.4:1 would be acceptable, if not preferred.

3) Weight. Our home-brew masts are only 1.5" thick, but heavy gauge Aluminium tube (off an old, large tent). They bend easily, and we only put up masts made from three sections (about 19'), with two sets of three guys. We are experimenting with some ex-military 2" masts but they only go up to about 12'. The J-pole I made was in 3/4" copper tube. 6' of it is quite a weight to erect; not impossible, but it takes some effort. And we get strange looks and comments from the un-initated about it ("got your cactus growing, Dave?").

4) Gain. Frankly, I'm not too bothered about gain. I'd prefer a low angle radiation first. The exercises we do are not usually for Dx use. We have other aerials for that.

5) Measurements. AS previously mentioned, I have an MFJ259B and a Kuranishi BR400. I usually stand well back, or raise the aerial above me when taking measurments.

6) Types available:  I'm not familiar with the Bow-Tie design (skeleton Bi-cone?). Can one be made sufficiently robust ? I'd like to try one anyway.

I had though about a coax-sleeve dipole (fed up the centre), but I had trouble getting the bandwidth, even with thick conductors. I've not tried using a copper tube for the radiator, yet.
I had also wondered about using a folded monopole main element and dropping the ground plane downwards.

I once tried to make a coax-dipole (the sort that connects inner to braid every so often). The instructions said RG58, but I was advised that it had problems. I never managed to get it going and binned the design.

Is there a chart for cutting (shortening) according to Length:Diameter ratio?

Many thanks to all who have helped so far.

WB2WIK de G8UYZ

Thanks for the suggestion.

Our ATC has two VHF 'bands'. Four channels in 149-150MHz and three more channels in 153-154MHz.

The aerials I have tried so far have not been able to tune both with a satisfactory VSWR. The only one that managed it was a 'Plumber's Delight' J-pole made with half-inch copper pipe and it is too heavy for our light-weight masts.

I have a Kuranishi BR400 VHF/UHF 'analyser' and an MFJ 269 ditto.

Thanks for the help.

But it is IMPORTANT that the youths concerned are not 'put at risk' by spikes (we have some serious Health & Safety rules). I had not considered a ground plane for that very reason, unless there's a way of using helicals for the ground plane, or shaping them round some way so they don't stick out, down, up or wherever.

Despite many hours researching, much hot soldering and bending, I cannot find a good working design for a simple vertical aerial for Field operation in the 149MHz or 154MHz region. I can get one or the other, but not both!

The aerial must NOT have radials (it's for a bunch of teenagers who are trouble on legs). The only exception is if the radials are folded round or down.

I have tried aerials made of Coax & Sleeve dipoles. The nearest I got was with a Slim Jim or a J-pole, but could not get the necessary bandwidth, unless it was really thick copper tube and then it got dangerously heavy.

Anyone got any ideas please?

No problem.
But if your aerial is aluminium, clean it up properly. Use graphite or copper loaded grease at any joints, and spray it with "acid-etch primer" first.
You should get this stuff at a motor-spares stockist, where it is used on some car bodies (an alternative might be a marine chandlers).

Then spray it. When dry, check it's allOK and mount the new black (or whatever colour), stand back and admire your work.

Hokushin HX-5, Cushcraft Ringo Ranger  or Diamond WD300.
I've used all of them at times and they are all very good. The WD-300 has the nicest finish and the price is competitive (well, in the UK!).
73
Dave
G8UYZ