Author Topic: diy limited space dipole  (Read 414 times)

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Offline m3vuv

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diy limited space dipole
« on: February 25, 2020, 02:11:07 pm »
Hi all,i live in a flat with no garden so am very restricted on antanna options,,for a number of years i used to join the barrometric net on 3.775khz,that was fine with a wire dipole at my old place but not possible here,firstly i had a 40ish foot 2" mast with a 2/70/6m colinear on it,i insulated the mast and shunt fed it using a 9-1 unun,i used all the copper water pipes as a ground/counterpoise,it works fine for contacts 2000miles plus but rubbish for inter G working,low angle radiation i suppose being a verticle,sevcond trie was using 2x 80m helicaly wound whips on a 3/8 inch dipole center,the swr is at 1.3-1 at 3.775mhz,its up at about 12 foot,it seems totaly rubbish on rx and tx,i have a 30khz 2-1 bandwith,the feeder is rg-213 about 70ft long,its had water in it in the past and the center conductor has gone black,would the coax cause such poor performance or do i look elsewhere?,ive ordered new coax but thought i would ask here,73 m3vuv
 

Offline ZigmundRat

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Re: diy limited space dipole
« Reply #1 on: February 27, 2020, 02:31:15 am »
Without knowing the actual layout, height, mounting options, and so on, it’s difficult to suggest a specific solution. That said, it sounds like your helical whip antennas are more or less of the mobile type and so physically quite short for 75M. These will have poor efficiency even though the match may be acceptable.

The other problem you have is the coax. Knowing it had water in it and that it’s corroded, I would chuck it in the bin. RG 8X, RG8, RG213- anything new will assuredly be better, and at least trustworthy. It could well be part of the problem and could also mask problems due to abnormally high loss.

Most any 75M antenna 12 feet off the ground will have you operating in NVIS mode, which will be OK out to maybe a couple hundred miles, but poor for DX as all the energy basically goes straight up. Your best bet may be a magnetic loop- but these are also problematic the lower in frequency you go, which is why you don’t see many options for below 40M. But they are physically small. There are various articles on home brewing these and there are a couple commercial options. Home brewing these is hard due to the voltages and currents involved. I can’t vouch for the company, but one example I found was at http://www.loop2er.cz/ - at least you can see some examples there.

Maybe this is of some help.

Cheers!
 

Offline m3vuv

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Re: diy limited space dipole
« Reply #2 on: February 27, 2020, 10:57:32 am »
I did try a 6meter diameter loop a while ago made from 15mm copper tube,it was tuned by a stepper motor running a 1000pf cap  thru a 20-1 reduction gear ,issue was the bandwith was only about 2kc wide,made it usless for ssb voice as the rig went into power foldback due to high swr on voice peaks,ive got new coax comming,im gonna try that,nvis is what im after,73.m3vuv.
 

Offline Wallace Gasiewicz

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Re: diy limited space dipole
« Reply #3 on: February 27, 2020, 01:36:50 pm »
I live in an antenna restricted neighborhood. I have two antennas, One is a big G5RV hidden in trees and the other is a 10 foot dia magnetic loop, which is currently inside.
The loop works better for me. it does 40-10 meters.
It is a homebrew and made of two inch copper pipe. Aluminum pipe would probably work just as well.
I only run maybe 400 watts. Reasonable bandwidth is maybe half of a band without retuning the vacuum cap.
Lots of folks run loops on SSB without any trouble.
Anything more than 40 meters is a problem but others have made loops for lower frequencies....
I would refer you to Groups.io for help in building one, I built one myself.
There are two groups which are helpful:

https://groups.io/g/MagLoop/topics

https://groups.io/g/HelicallyLoadedMagLoop/topics

Wally KC9INK
 

Offline m3vuv

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Re: diy limited space dipole
« Reply #4 on: February 29, 2020, 07:40:07 am »
well i replaced the coax with rg-213,rx  seems a lot better but tx inter "g" is rubbish!,the swr is now 1.2-1 it was 1.7-1 with the old coax,if i used the water pipes here as a counter poise / ground ,would that likley help?,im thinking of connecting the screen of the coax to them,there is about 100ft of pipework here.m3vuv 73.
 


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