[UK-Contest] Inverted V help (Ian White, G3SEK)

[Andy Summers]

Hi,

Is it OK to talk antennas on this reflector?

I'd disagree that bent & folded wires necessarily increases the risk of
feeder radiation. RF on the braid can only occur either by the outer RF
'skin' being excited by the antenna directly, or due to the transition from
an unbalanced feedline to a balanced antenna. As far as the latter is
concerned, bending the wires can only make the antenna less balanced, can't
it? From this point of view baluns are useful regardless of whether the
antenna is bent or not. Conversely, the balun does not solve the direct
pick-up problem, it merely isolates the braid outer at the feedpoint. In
this instance antenna assymetry does come into play. Avoiding resonant
lengths of feeder is therefore a good idea if your antenna is laid out
physically assymetric with respect to the feeder.

Over the years I've come to mis-trust current baluns, especially the
commercial ones. Thinking about what you're trying to do, it can only be to
choke RF from going down the outside of the braid at the balanced/unbalanced
transition. In this respect I totally agree with Ian that the simplest way
of achieving this is to make an inductor out of the coax. I prefer to use
the 'G5RV style' balun from Ferromagnetics, which achieves the same end
result without the extra coax. It might therefore be lower loss. They
advertise in the back of Radcom.

Having said that, I reckon it probably makes zero difference on a low
dipole, but it might make you feel better! In my youth I had a pair of
phased dipoles on 80m at about 35ft. Adding baluns did nothing for my
signal.

73,
Andy.

> -----Original Message-----
> Date: Wed, 19 Feb 2003 07:54:37 +0000
> From: "Ian White, G3SEK" <G3SEK at ifwtech.co.uk>
> To: uk-contest at contesting.com
> Subject: Re: [UK-CONTEST] Inverted V help
> Message-ID: <Mzf1VEF9gzU+EAii at ifwtech.co.uk>
> In-Reply-To: <004c01c2d797$3d00eea0$0300a8c0 at zebidee>
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> Reply-To: "Ian White, G3SEK" <g3sek at ifwtech.co.uk>
> Message: 1
>
> Alex Allan wrote:
>
> >I agree with Stuart, omit the balun and try to resonate the
> dipole on it's
> >own.  Provided non of the folded bits are at an acute angle to
> the main run
> >(i.e. not less than 90 deg) I would expect to get it resonant
> and less than
> >2:1.
>
> But having got the dipole resonant, consider putting the balun back,
> because the bent and folded wires make the whole thing a strong
> candidate for feedline radiation.
>
> According to your tests, your existing balun looks dodgy. A coiled-coax
> balun is so simple, it can't go far wrong - for 80m, about 20ft of coax
> in 6-8 flat turns (according to the ARRL Antenna Handbook).
>
> --
> 73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
>                             Editor, 'The VHF/UHF DX Book'
> http://www.ifwtech.co.uk/g3sek