Doublet

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Doublet

Postby mike43 » Wed Nov 20, 2019 3:09 pm

I am a Shortwave listener and am in the process of changing my antenna system (again) as an experiment. I want to erect a doublet, centre fed with 300 Ohm ladder line. I have an awkward shaped small garden so the antenna wire will be about 120 feet in length, and 20 feet or so in the air 99% above the roof line. One side of the centre will be 54 feet or so straight, the other side owing to the shape of the garden will (have to) Zig Zag. I have a couple of questions here. Will the Zig Zag part of the antenna be a problem for receiving, also one part of the straight section will be about 4 feet above the telephone cable coming into the house, will that cause any problems as far as interference goes?. Also this part will be about 6 feet above the Sky dish, same question, will that be a problem with interference?. I am not expecting any miracles here, just want to listen around as I have done for years. Thanks upfront, I will be grateful for any help from the forum members.
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Re: Doublet

Postby Allan » Sat Nov 30, 2019 10:43 am

I use a Doublet ,I live in a detached Bungalow with a 20 foot mast at each end and like u I also have a Telephone cable running to the roofs apex. One end of the aerial fits to the top of the mast runs across my roof to the other 20 foot mast which just under the Doublet is attached by a pully and on the same mast, Is my Satellite Dish, 48 else TV Aerial and at the side is the Telephone cable which is secured to the weather board. My aerial wire leaves that mast and goes out to a tree in the garden then it has a 90 bend to another tree. In the middle is fed with 300 ribbon into a Ballun coax into the shack… It works great and no unwanted interference in my bungalow while I transmit about 100 wats, Been like that for about 15 years.Incedently I also have a full size G5RV.. The Doublet is far far better on TX/RX
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Re: Doublet

Postby mike43 » Sun Dec 01, 2019 3:22 pm

My thanks to Alan and Chris for assistance. My layout for the doublet is roughly the same as Allan's more or less minus the balun. The 300 ribbon goes under the eaves into the loft and down to the shack. About the balun...I have a 9-1 unun left over from a random wire antenna that was around the garden. Where would I attach the unun as it needs an earth stake doesn't?. The unun is still outside my bedroom window by the earth stake from my earlier random wire, I will be back to the earth stake again if I use the unun?. Would I be better routing the ribbon alongside the wall of the bungalow instead of through the loft to the unun & earth, about 4 feet of RG8 and then through the hole in the window frame to the ATU. Once again thanks for help..Mike. PS..how would the 300 ribbon be attached to the unun, would it be the same as treating the ribbon as the coax one side to the unun, the other side to the earth stake, and then via the unun SO239 to the ATU. Apologies for my lack of knowledge here.
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Re: Doublet

Postby Chris P » Mon Dec 02, 2019 8:20 am

In theory a doublet requires a BALUN between the balanced feeder and the unbalanced (Coax) feed to the receiver . Feeding a balanced feeder such as a ladderline through a building is not a practical proposition . Again in theory if the RX has a 50 ohm impedance input the balun should have a 6:1 ratio . In addition an ATU between the coax feed and RX input is also required .

Hope this helps
Regards Chris aka G8FFF nipper or tazmin88
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Re: Doublet

Postby mike43 » Mon Dec 02, 2019 11:02 am

Thanks for that Chris. I will have a go at doing a bit of re-routing instead of going through the loft. I know you stated "balun" in your answer, would a 9-1 unun do the trick for receiving only. If so, the unun has as you know a connection for antenna and another connection for earth (ground stake) and an SO239 for the coax to go through the window frame to the ATU. I am a bit puzzled as to how you would attach a 300 ohm ladder line (balanced feeder) to the balun/unun. Random/long wire has only one wire, so how would a two legged feeder connect to the balun/unun. Any help would be appreciated.
Thanks..Mike.
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Re: Doublet

Postby Chris P » Wed Dec 04, 2019 5:34 am

Try the following link

https://www.balundesigns.com/blog/balun ... tennas-fed.


You could try connecting one side of the ladder line to the long wire connection and the other to the earth connection of the unun but the results would not be as good as a true balun
Regards Chris aka G8FFF nipper or tazmin88
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