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 Open Feedline Doublet Antenna

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Victor
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Victor

Call Sign : 26-CT-3228 Ham M7VIC
Posts : 787
Times Thanked : 53
Join date : 2019-11-10
QTH or Location : Bedford
Equipment Used : Various

Open Feedline Doublet Antenna Empty
PostSubject: Open Feedline Doublet Antenna   Open Feedline Doublet Antenna Icon_minitimeMon Dec 09, 2019 10:37 am

Hi all,

I know many members here like to experiment with antennas and there are many Hams who will have experience of different antenna systems. So probably the best place to ask Wink

I've experimented with various antenna designs over the years from simple dipoles to magnetic loops with varying degrees of success, (and failures!)
But it all jogged a memory from my teen years, (yeah, back in 19clickity-click), of an old Ham and his setup. He too began experimenting with antennas and after years of "playing" settled with a doublet running over his house spaced at the roof apex and angled down either end of his gardens. I have a similarly sized plot today where my property is centred so the front and back gardens run the same length, (some 100ft or so total run including the house).

He called it "the last antenna I'll ever need" and I remember him being able to go to any band, at any time and within minutes of calling, getting an answer back!
(I also remember him "starting it up" he would jokingly say whilst winding the handle of a roller inductor on his antenna matcher!)


Here's where someone might be able to help...

I can't ask the old Ham.... I'm sure his key fell silent many years ago.
I get ladder line, or open wire feeder and love that you can make it yourself.
There seems to be misconceptions about what a Doublet antenna is, some think it's a dipole, others call it a double Zepp amongst other things. This hasn't helped my search.
But....

I've had limited success on finding details about balanced antenna matchers(?!)

Most seem to revolve around simple "atu" circuits and poorly tacking a balun to it. (Commercial and Homebrew) Others are over complicated affairs using exotic tuning capacitors which are rare as hen's teeth and ridiculously overpriced.

I do have vague recollections of a L-C circuit at the rig end with the inductor coupled to another L-C circuit at the antenna end, that end having a tapped inductor for different band operation(?), or maybe wide range match switching(?)

If only I could remember what the circuit was called I'd have better luck!
(Google searches insist on adverts for products that have nothing to do with what I'm searching for)
I've dug through my old Practical Wireless collection, 1950s through 1970s and found nothing.... I know I haven't imagined the circuit Wink

Anyone able to prod my old grey matter with suggestions?

Much appreciated,
Victor


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Victor
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Victor

Call Sign : 26-CT-3228 Ham M7VIC
Posts : 787
Times Thanked : 53
Join date : 2019-11-10
QTH or Location : Bedford
Equipment Used : Various

Open Feedline Doublet Antenna Empty
PostSubject: Re: Open Feedline Doublet Antenna   Open Feedline Doublet Antenna Icon_minitimeMon Dec 16, 2019 8:16 am

Found it!......Link Coupled Balanced ATU Wink

Had to trawl through God knows what antenna articles to find that little b*gger!
(Google is no help if you don't know exactly what you're after!)


Well thanks for all the help..help....help....elp....elp....
Where did everyone go....go....go....go....o....?


Hee, hee Very Happy Joking aside, took me a while to find that snippet of info from a vague memory of something from years ago.

Had more questions.......suppose I better keep "mining" that old internet!


_________________
Theory means that we know everything, but nothing works.
Practice means that everything works, but we don't know why.
Often, however, theory is closely related to practice.
It means nothing works and nobody knows why ...
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43CT016
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43CT016

Call Sign : 43-CT-016
Posts : 51
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Join date : 2019-11-17
QTH or Location : Perth
Equipment Used : iCom IC-7610/IC-9700

Open Feedline Doublet Antenna Empty
PostSubject: Re: Open Feedline Doublet Antenna   Open Feedline Doublet Antenna Icon_minitimeMon Dec 16, 2019 9:47 am

I've always viewed the difference between a doublet and dipole as being the feed line...a doublet being fed with ladder line and a dipole being fed with coax.

Ideally, if it's fed by ladder line you want a tuner with a balanced feeder input, or if you're changing to coax to feed the tuner or radio, then a balun to match to the coax's
impedance. When I ran a G5RV I just fed it via a tuner, but never got great results probably because I couldn't get it high enough. However, I've long since moved to horizontal delta loop, again fed via ladder line to a tuner, and then coax back to the radio.
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Victor
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Victor

Call Sign : 26-CT-3228 Ham M7VIC
Posts : 787
Times Thanked : 53
Join date : 2019-11-10
QTH or Location : Bedford
Equipment Used : Various

Open Feedline Doublet Antenna Empty
PostSubject: Re: Open Feedline Doublet Antenna   Open Feedline Doublet Antenna Icon_minitimeMon Dec 16, 2019 10:06 am

Hi Jeff,

Yep, pretty much how I thought of it, the ladder line making the difference.

Haven't 'played' with CB radio for 30 years and my Ham experience was even longer than that as a boy, so I'm pretty vague on specifics.

Still, been really enjoying finding out technicalities even if I do have to dig past 'misinformation' Wink

Wanted more than anything to build my own antenna solution, getting more of a thrill from that than purchased solutions.
(Heck, even been looking at getting my "ticket" before I pop me clogs!)

Cheers for your experience, much appreciated,
Victor

_________________
Theory means that we know everything, but nothing works.
Practice means that everything works, but we don't know why.
Often, however, theory is closely related to practice.
It means nothing works and nobody knows why ...
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26TC62
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26TC62

Call Sign : 26TC62, 26CT2817
Posts : 142
Times Thanked : 29
Join date : 2019-06-27
QTH or Location : Portsmouth
Equipment Used : Yaesu 891, 10m pole, wire 5/8, T2LT

Open Feedline Doublet Antenna Empty
PostSubject: Re: Open Feedline Doublet Antenna   Open Feedline Doublet Antenna Icon_minitimeTue Dec 17, 2019 12:38 pm

People tend to talk about dipoles to mean two-legged antennas which have some resonant quality, usually a half wave, and a doublet to mean a two-legged antenna which is not necessarily resonant. If you feed a doublet with open-wire feeder and match the whole lot at the shack end with a link-coupled tuner, or another design which works with a balanced load (eg Z-match), it minimises SWR-related losses. Dipoles generally being matched antennas, it is more of an option to use a 1:1 balun and coax - although if you want to use this arrangement on more than one band, it can be lossy as the SWR on other bands may not match acceptably to 50 Ohms.

People sometimes find poor results trying to use balanced feeder and a balun, and then an ATU. The best way to match a balanced feeder is with a proper balanced tuner which cuts out the need for a balun - they aren't widely available commercially but are easy enough to build even for higher power levels.
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Victor
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Victor

Call Sign : 26-CT-3228 Ham M7VIC
Posts : 787
Times Thanked : 53
Join date : 2019-11-10
QTH or Location : Bedford
Equipment Used : Various

Open Feedline Doublet Antenna Empty
PostSubject: Re: Open Feedline Doublet Antenna   Open Feedline Doublet Antenna Icon_minitimeTue Dec 17, 2019 2:26 pm

Hi "26TC62", (my apologies, couldn't find your name)

Spot on!
What you've just said would've saved me a walk up the 'garden path' a week ago Wink


I've since come to the conclusion that "mainstream" chat is as helpful as Hams blasting 1kW on 80m locally wiping out the whole band for us poor SWLs.....those who shout loudest aren't necessarily correct!

Luckily it seems the QRP operators of the world still cling to viable knowledge.
That's where I have found the information I was after and a detailed description of a matcher that doesn't require exotic variable caps.
Scrounging the parts together as we speak Very Happy

Nice to hear someone quote non-resonant antennas, seems it's sacralige these days Wink

Cheers for that and all the best to you,
Much appreciated,
Victor


_________________
Theory means that we know everything, but nothing works.
Practice means that everything works, but we don't know why.
Often, however, theory is closely related to practice.
It means nothing works and nobody knows why ...
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