low band antenna wanted

deaglan169

i was wondering if someone could help me im looking for a lowband antenna for the range roughly 70-88mhz if possible centred around 84mhz, something like the sigma folded dipole or green vertical stick antennas there must be plenty lying around from old taxi ranks and emergency services upgrades

watty

A discone works nearly as well.

Also http://helmut-singer.de/ has had 70MHz to 80MHz end fed vertical dipole including a multisection mast and guys all packs into a canvas bag.

deaglan169

hi watty i cant seem to find that particular antenna on that site any way you can send a direct link to it

watty

No they don't seem to have it at the minute. Though some of the Clansman stuff might be cut to length. It's a very awkward site to "deep link" due to how they use frames.

Antenna

You would probably find a lot of disused low-band "sigma folded dipole " (the 68-87MHz varity) around the country on business premises etc, someone might let you take away one for free.
Huge decline in Low band 2-way radio use by business people since mobile phones took off

hamradio

I got one of them sigma- type folded dipoles years ago and kept it in the shed for future use. Unfortunately the feeder cable to the centre bit was found to be broken when I pulled the antenna out of the shed to get ready for this years tropo-season. I opened the centre bit and did some investigation what sort of transmission link the manufacturer used to connect a 50 ohm cable to a 240 to 300 ohm folded dipole. There was a lot of resin in the centre bit and it was very hard to get it out. To my amusement I found only a couple of inches of copper wire....
I would like to know how to reconnect the antenna to a 50 ohms cable. So far I got a "Variobox" from a local retailer, together with two glands which work as inlets for the two dipole branches. After some fiddeling it fitted perfectly. I made up a transmission link out of RG58U. On the first trial bit I used 141 centimetres and rolled it up. It looked great in the box and I connected it up so that it works as a 1:4 impedance matching transformer. When I put it up the mast and tested the antenna there was a SWR of 1:1.6 on 70.350 MHz. There was a perfect match on 88 MHz with 1:1. I made up several other links and soldered them in one by one. All had different lenghts. I did not get any perfect match again until I had a part with the lenght of 151.5 centimetres in. This time the SWR on 70.350 was still 1:1.7. But the perfect match had moved to 80 MHz-1:1 there now. I did some calculations and came to the conclusion that the SWR would be 1:1 on 70.350 MHz when I increase the link to 156 centimetres. But after installing the new bit I found no perfect match anywhere- and the SWR on 70.350 MHz is still around 1:1.7.... GRRRRRRRRRRR!!!!!!!!!!!!!!
Reception with the dipole is great- I get the cluster packet EI7WDX from Bray with 55 all the time on 70.325 MHz in East Mayo. And some commercial stations around 79 MHz. Of course the local boys on the call channel 70.265.5 MHz....
BUT- I am not happy with the SWR- it MUST be better!
The workshopfloor is littered with cut- up bits of RG58U, everywhere are rests of electrical tape and solder. My nerves are in bits after so many failed trials. I do not have an antenna analyzer- this would make the job probably easier.
Did somebody ever come across this problem or knows a solution which would fit into my Variobox, giving the antenna the perfect match on 70.350 MHz ?
A QSO in any mode and a QSL from square IO53 would be your reward-hi!
73 de Michael

watty

3:1 is 0nly 25% mismatch
better than 2:1 is fine. I'd be happy with 1:1.7 It could be the radio, the aerial or whatever not your transformer. For best results use a 2:1 ratio transforner (change 250 -> 50 , or 300 -> 75) and then a current balun.

I must build up my 4m convertor. I only have an FM scanner for 4M.

You will have hardly any difference in TX range or reception quality between 1:1 and 1:2 SWR.

Remember a 20dB in line atenuator always gives a perfect match, but isn't at all what you want. A low loss match at 2:1 is better than a 3dB loss matching circuit that gives 1:1

hamradio

Thanks for the answer.
It must be my German genes which make me working towards the perfect match-hi.
I will get a bit more coax and will try again in a while..I saw the SWR being perfect on 2 frequencies- so it must be possible to have it ok around the 4 metre band centre frequency as well. But it looks very much like a lucky shot in the dark.
At the moment I have a Philips FM 1000 on the folded dipole.
Got a Spectrum transverter some time ago and will use a FT290 as a driver. Antenna will be a Moxon rectangle with a cheap rotor, all fixed on top of an old TV mast. Hope to have it ready for Easter.
It should be possible to have a simplex QSO between here and LK when you are ready with your transverter.
Long time no hear from you via the midlands repeater. I had a few chats with you last spring or summer. Are you still driving towards Dublin in the mornings?
73 de Michael/ei3gyb

watty

No. I'm in Limerick now.

Your aerial could be slightly short for 4m hence the "perfect match" at 80MHz. It's really not worth spending more effort. You might break it.

Normally I could access R3 and into SEARG. I think I need to check a feed cable.

hamradio

The aerial is indeed a bit too short for the 4 metre Ham band. Its is exactly 361 centimetres from one end of the dipole to the other one.
But there is an imprint on the antenna: "68 to 88 MHz" . And as a producer there was "Ireland Communications LTD" written on the centre bit. It is exactly the same antenna you see on many guarda stations, fire brigade bases or factory offices. Most of the time not in use anymore, with the cable jut being cut-off at same stage. There must be hundreds of them around the country. My aerial was in use at a private Cork City based bus operator in the late eighties/early ninties and was just dumped at some stage during renovation work.
I think you are right- I reached the best possibility with my set-up. I only got worried about the warming-up of the rig. After a 30 minutes chat with 30 watts it is quite warm at the back. A SWR of 1:1 would have probably left it a bit on the cooler side. I would not mind if the price for the 'perfect' match would be some hearing loss.
But here I am again riding a dead horse....
May be I get some copper pipe and use the pipe bending machine to make a folded dipole with the right size for the Ham four metre band.
In the meantime I stick to the current antenna and keep the TRX on the short side.
CU on any band!
73 de Michael

watty

If you work out the extra reflected power even at 2:1 it won't make the rig warmer than 1:1 Run rig into a dummy load for same time. It will get just as warm.

I have a military rig. It takes 90W to give 22W RF

It takes over 24W on receive and 48W on SSB without talking!


Talking to you coming back from Dublin the Icom 2800 would nearly burn my ankle. It even has a fan! (50W 2m, 35W 70cm)

Antenna

hamradio wrote: »

Unfortunately the feeder cable to the centre bit was found to be broken when I pulled the antenna out of the shed to get ready for this years tropo-season. I opened the centre bit and did some investigation what sort of transmission link the manufacturer used to connect a 50 ohm cable to a 240 to 300 ohm folded dipole. There was a lot of resin in the centre bit and it was very hard to get it out. To my amusement I found only a couple of inches of copper wire....
I would like to know how to reconnect the antenna to a 50 ohms cable.

They were called the SDH80 (centred on 80MHz)

The original length of coax was not all 50 Ohms
Look at the coax and you will see two pieces of different coax joined together in a mold. Most of it the normal 50 Ohms.
The last bit to the antenna however was a higher impedance (something over 100 Ohms) and used as a quarter wave transformer

http://www.arcticpeak.com/antennapages/quarter-wave_transformer.htm

hamradio

Indeed I remember the molded bit. You still can see it along existing feeding lines when you look very careful along the mast. When I saw it first I thought it was some sort of a joint. The joint was not possible at the centre bit- that was sealed with resin and the lid was glued on the body. I thought the antennas were sold with the bit of coax and then joint to the feeding cable the way I found it, with a bit of weatherproof sealer tape over the joint. I remember that it was badly damaged when I found it on the site- cable bits all over the place. So somebody must have removed part of the transformer cable...It explains also why I did not find anything inside the centre bit when I opened it pointing towards a matching bit. Thanks for solving this puzzle for me.
I used the formular: 1/2 lambda multiplied by the velocity factor 0.66. So using 70.350MHz as the centre frequency I came to the lenght of 140.5 cms. When you use LA8OKA's calculator, it is 140.7 cms. So I was bang on with my matcher. Only trouble- as Watty pointed out- the antenna is a bit too short for the HAM 4 metre band. Or we put up with a SWR not neccesary being the best one.
Well- that's life, I presume. Nothing is perfect...
About the TRX getting hot: I never had any trouble with a hot rig when I was taking to you, Michael. I used a Yaesu FT857D- probably 50 watts to the Midland's repeater. Only slightly warm after an hour. The fan is going all the time- keeps it cool from the word go.
I probably find you on the LK repeater in the future- may be after the NX on Monday! 73 to all!
Michael

watty

FT857 a much more modern efficient rig than the 4M rig

I considered a FT857 or FT897, but as I have the Icom 2800 in car, a Racal Syncal30, an Alinco dual band FM handheld and the tasty FT817ND, I bought a very clean FT101ZD MkIII with WARC and either the AM or FM board (not sure which yet) instead, Cheaper and looks nice in shack. Also I can't misplace it.

If I get rich and famous maybe I'll get an FT897.