Antenna

[VK2XSO]

A quick mental task for those who like to home brew antennas.

I have an antenna. Or rather, I had an antenna. Lets just say it had a small altercation with a helicopter and the antenna is now gawn !
All that is left is LDF4-50 coax dangling in the air.

Your task should you choose to accept it, is to make that loose piece of heliax into an antenna.
The frequency doesn't really matter since the antenna can be scaled for any band. But if you insist, it's for 147MHz.

There is a limited amount of coax hanging free to make the antenna. A little over 2m.
It is not possible to take a soldering iron up to the coax. It is possible to take other small hand tools like a knife, cutters or screw driver.

Any ideas ?

[VK2MUS]

Your task should you choose to accept it,

well I am always in when it comes to making antennas with whats left laying around the back yard but could you move that tower out of the flght path of the helicopters before I climb up there.

[VK2HRX]

Simple. Make a Flower Pot. http://vk2zoi.com/flower-pots/

Can be made for a few bands or even multiband. All you need is a ruler, knife, tape and something like a stick to hold it up. I have written an article for a 4WD club on creating a usable anntena from your remaining coax when your UHF CB antena is demolished.

Next!

Compton
VK2HRX

[VK7DB]

Fold the shield back and make a 1/4 wave?

[ZL1RS]

Simple. Make a Flower Pot. http://vk2zoi.com/flower-pots/

... and from the flower-pots web page:

Other suggestions made at the time included making a fake patio umbrella ...

In my mind's eye there is a phased array of patio umbrellas (a.k.a top loaded verticals) on one of the upper HF bands for the avid ham living in a "no antennas" zone

73, Bob ZL1RS ... (obviously with too much time on my hands!)

[VK4KGW]

A simple 1/2 wave dipole would work although there are many easy to build semi invisible "things" that will work , striping the braid back and trimming to the dipole would take seconds

[VK2XSO]

ok... I should refine the question a little.
How do you make the kind of antenna suggested ?

Basically I'm building and testing the suggestions. Only takes me a few seconds with a knife and i have LOTS of heliax to experiment with.

This is what a quarter wave vertical does with a quarter wave ground plane.
http://home-and-garden.webshots.com/pho ... 1318UOPrEx
I've got one save somewhere for a 1/4 (5/8) wave without a ground plane
and a 1/4 wave with a wire run back to form a dipole. (Though I can't remember what that one looks like of the top of my head)
I'll find them and post them here.

If figure there must be some better ideas out there than my own, hence my asking.

[ZL1RS]

The sleeve dipole was popular some years (decades) back. In the 70's I made one but substituted the copper pipe sleeve with 4 lengths of brass rod to simulate the sleeve. It looked quite tidy and worked fine as a local 2m antenna ... working stations around the city with a converted Pye Reporter (a commercial mobile RT with valves producing 4W of AM rock-bound on 144.2 ... weeeee)

sleevedipole.GIF (10.17 KiB) Viewed 3214 times

This could be assembled at the top of the tower with the tools you suggest after suitable dimensions were found (radiator length and sleeve length will be a bit less than 1/4 WL each). The PVC pipe radome will alter the tuning of a bare sleeve dipole. Also, a short length of PVC should be tested in a microwave oven (with a cup of water) for a minute on high to ensure it is "good" PVC at RF. Of course other rigid plastic pipe or fiberglass tube could be used in place of the PVC pipe.

73, Bob ZL1RS

[VK2XSO]

Thanks Bob,
I'll build that one and test it. It's the kind of thing I'm looking for.
I'm not really going to worry about a PVC radome. It's more likely I will tape up the antenna and it will hang inverted.
A roll of self amalgamating tape and UV tape will do the job nicely.


I'm not a fan of the microwave oven pseudo test. There is a little difference between 146MHz and 2450MHz in absorption aspects in frequency and power.
I would just use the PVC tubing anyway unless I had cause to suspect it create problems.

I think the microwave oven PVC test is a bit of a myth. But it might be worth conducting some experiments to confirm or debunk this idea.

[VK4WDM]

Sleeve dipoles are a very useful antennas for VHF and UHF and were, as Bob says, very common in the 70's and early 80's. I have used one on 23cm and it worked quite well.

73

Wayne VK4WDM

[VK4WDM]

I forgot to say that I really like to see promotion of this sort of simple activity, especially for the newbies. We often forget that this is amateur radio and there is nothing wrong with "amateurish" approaches to things like making a usable antenna out of materials at hand to get a signal on the air.

I forgot to take a 70cm antenna with me on my last trip away. I made one out of a metal coat hanger and it worked fine.

On another trip I lost an HF wire antenna and feed line. I went to a local hardware, got a length of ordinary electrical twin lead, split it down the middle to produce a 40m dipole, used the unsplit portion as the feed line, and worked plenty of stations with it.

73

Wayne VK4WDM

[VK2XSO]

I had hoped to have tested the prototype by today so I could publish the frequency response.
I have not had the time to do it and I did want to get the first working example up by tomorrow.
But I'll have to push it back a week or so.

[VK2XSO]

So I tried a couple of simple ideas today.
A quarter wave hanging out the top of the coax. - yeah well this works, but you wouldn't use it if you didn't have to. SWR was sad to say the least.

With a 1/4 wave skirt, it worked a little better. Good enough to put into service with an SWR of about 1.8:1
A 5/8 with 1/4 skirt was a little better with an SWR of about 1.5:1

I started with some other options too, like an isopole.
I didn't get far before I ran out of light today, but a 5/8 exposed core with a 1/4 section of coax insulated shield cut into the end of the heliax worked
really well. Much better than any other combination. At least in terms of return loss. I'd need to check the radiation pattern to be sure it was radiating with any sort of useful pattern.

===========-====------
^ gap in shield

Another combination that showed promise was the same 5/8 core over 1/4 insulate over 1/4 wave exposed core with a jumper across the gap.
I intend to use a piece of inverted coax here next.

===================----====------
^----^ jumper wire

[VK2XSO]

And the more I experiment, the more I realise how much I don't know about antennas.

A test I did today showed a good result.
A half wave length of coax which is shorted at both ends.
The shield was then cut half way and fed directly at this point.
This on it's own works well. Phasing them works better

I tapped the main coax directly at half wave intervals and fed each of these coax dipoles mounted up directly against the insulation of the main coax.
I'll try some tests with it tomorrow if time permits.

[VK4ABW]

The 6M9KHW is up.
50ft high, 50ft long. sweet

I'm using a G800DX rotator outside the tower on a small platform to turn the 6M9 thru 450deg. There is another G800DX rotator inside the tower which rotates an 'inner' 50mm pipe for my 2M12 yagi (pending delivery). Both antenna's will be fed with 7/8 hardline with FullStop connectors and then down to 1/2 Flexline into the amps. LMR400 is used to get past the rotators, direct to the feedpoint.

Gary
VK8AW / VK4ABW

[VK4ABW]

Part 2. A few pics showing the drive system.

Gary
VK8AW / VK4ABW

[VK2KRR]

Excellent! good stuff Gary.
Grab another pic when the 2m yagi is up.

You should try video some of the openings up there for youtube, im sure other southerners would be interested in having a look.