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What kind of antenna is this?!?

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What kind of antenna is this?!?

Post by davegsm82 » Fri Nov 14, 2014 7:51 pm

I've had this antenna behind the sofa for some years now, I can't remember what it came from but I was thinking maybe it could be useful for ham-ateur frequencies, that was until I opened it up and found it's not a regular single pole antenna.

Inside is a piece of what I believe to be 300 ohm flat feeder, the sort that FM antenna's used to be made from that you got with your AIWA hifi for example.

It's not as straightforward as that though, both the active element and the braid of the cable are connected at one end of the feeder cable, the active side is 335mm long, the 'ground' connection is 525mm long. after the Active side terminates, there is a gap of about 10mm, then it carries on (unconnected) for about another 655mm.

I understand there are some clever antenna designs out there but this one is beyond me, can anyone shed some light?

20141114_184435 (Custom).jpg
Braid and center core connected here.


20141114_184445 (Custom).jpg
Center core is notched out here...


20141114_184457 (Custom).jpg
Then the 'ground' stops and the center carries on...

 
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Re: What kind of antenna is this?!?

Post by Niall » Fri Nov 14, 2014 8:28 pm

Appears to have some similarities to this:

http://www.hamuniverse.com/2meter300ohmslimjim.html

Although the original designer of the Slim Jim, Fred Judd, didn't recommend the version made from 300 ohm twinlead.

 
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Re: What kind of antenna is this?!?

Post by Terrykc » Fri Nov 14, 2014 9:07 pm

If I've interpreted your post correctly, it should look like this:

Dave's Feeder.png
Dave's Feeder.png (3.05 KiB) Viewed 1408 times


My first thought was it might be a type of J pole antenna but you've got a bit too much! See these examples:

http://www.hamuniverse.com/jpole.html

http://www.n7qvc.com/copper-cactus-dual ... a-project/

I can only assume it is supposed to be some sort of multi-band antenna. Note that, to a reasonable approximation, the sections of the active wire are in 2:1 ratio and the grounded side is ~50% of the total of the overall length. However, beyond that, I'm stumped!

 
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Re: What kind of antenna is this?!?

Post by davegsm82 » Sat Nov 15, 2014 12:13 am

Hi Terry, Yes you've got it exactly right. I can't figure it out either!

I would like to rearrange or at least re-use the white PVC pole which houses the 300 ohm strip for 144-146MHz as I'm taking an interest in amateur radio and thinking of taking my foundation licence.

At the least I have most of the starting points, i.e. the pole (which is water-resistant at least!), a length of 50ohm coax and a BNC.

Niall, I've seen similar antennas but this one looks different, it's not shorted at the bottom and is bottom fed instead. Weird!

Dave.

 
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Re: What kind of antenna is this?!?

Post by Niall » Sat Nov 15, 2014 12:58 am

Yes, I had missed the fact that a whole section of one side of the ribbon is missing. Never seen anything like that. Did it look original, or has it possibly been cannibalised?

You could fit the deprecated ribbon slim jim into the pole, or you should be able to get a 5/8 + 5/8 collinear for 2M into the available length - might need to add a whip section to the top. There are loads of designs online.

 
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Re: What kind of antenna is this?!?

Post by davegsm82 » Sat Nov 15, 2014 1:15 am

I'm happy to scrap the twin-lead coax, what dimensions would I need to use for the antenna you've described?

I don't think it had been modified, I can vaguely remember removing it from the building myself, I think it might have once been part of a pager system but I have no idea what frequency.

The hole at the bottom of the pole is only about 10mm in diameter, which is about the width of the twin-lead. so that's about the width of the space I have available.

Cheers, Dave.

 
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Re: What kind of antenna is this?!?

Post by davegsm82 » Sat Nov 15, 2014 1:18 am

p.s. The antenna tube is actually a little longer than the overall length of the ribbon antenna, the whole thing is maybe 1.4M long, so there's a little more space to use.

Dave.

 
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Re: What kind of antenna is this?!?

Post by Niall » Sat Nov 15, 2014 1:55 am

Here's a dual band design which should fit:

http://www.m0zpk.co.uk/2m-144mhz-70cm-4 ... collinear/

Not tried this myself.

I have an old Practical Wireless with a design I have used at various frequencies - I currently have a 4M version made from an old CB antenna in the loft. I found a copy of it online:

http://g0pjo.cloudapp.net/Content/Images/Ringbase/Ringbase69.jpg
http://g0pjo.cloudapp.net/Content/Images/Ringbase/Ringbase70.jpg
http://g0pjo.cloudapp.net/Content/Images/Ringbase/Ringbase71.jpg

 
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Re: What kind of antenna is this?!?

Post by davegsm82 » Sat Nov 15, 2014 8:40 pm

By an abolsute twist of fate ive managed to find out wjat frequency this antenna was intended for.

Visiting the scrap yard today for some bits for my 283,000 mile golf, I spied some metal boxes in the corner and remembered that they were the type to which this antenna was attached.

The units were 'aid-call' medical pager units, for people bed-bound on hospital wards.

The main units contain a very large 4x figure-8 led display and a receiver PCB. I purchased all 8 displays and pcbs from the scrappy, upon inspecting the transmitters and receivers I have ascertained that the magic frequency is 173.500mhz :-D

Obviously this is no good for ham use, but I do now have some very high quality FM tx/rx pairs and some large displays to play with.

Still doesn't explain how the antenna works.... oh well. :aab
Dave.

 
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Re: What kind of antenna is this?!?

Post by Refugee » Sat Nov 15, 2014 10:07 pm

That is a license free telemetery band so they can be used. I worked for a firm that used them on early wireless alarm units.

 
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Re: What kind of antenna is this?!?

Post by Niall » Sun Nov 16, 2014 6:06 pm

I would adapt the ring base design for your pole by taking a length of copper wire (stripped from old twin and earth house wiring cable would be ideal). Cut enough for the half wave top section (0.98M for 145.5 Mhz centre frequency) and enough extra to wind the coil according to the article. Drill a hole big enough for the wire at an angle upwards into the pole about a metre down, then pass the wire into the pole until you reach the 0.98M mark, then wind the coil onto the outside of the pole.
Connect a length of 50 ohm coax, centre to the bottom of the coil, braid to a 0.49M length of wire to form the base ring.
When the adjustments have been made for best swr per the article, cover the coil and coax connection with a length of adhesive heatshrink, preferably white to match the pole.
Should be a good bit of pole left at the bottom for a mounting clamp.


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