[UK-CONTEST] Inverted V help

[Bob Henderson]

John

You have already had some good advice from other contributors but I'd just
like to add another two penneth.

1.  Forget the balun altogether.  It is probably more trouble than its
worth.  I have worked the world with inverted vee dipoles on the LF bands
for more than 30 years and have only ever been disappointed when I have used
baluns to feed them.  The purists will cringe, but just attach your coax
directly.  I'm no purist I'm a practitioner.

2.  With roughly 66ft of wire in each leg, sweep the dipole with your MFJ.
You will find a dip somewhere between 3 & 4 MHz.  It will be very unlikely
the dip will show a good SWR because your feed impedance at resonance will
be anything but 50 ohms.  However, the resonant point of the antenna IS at
the frequency where the MFJ shows minimum SWR despite this SWR looking none
too good.

3.  Bearing in mind that you will not get a 50 ohm feed impedance with the
dipole low in height and bent around your garden you will need to do
something to enable you to get power from your rig into the antenna.  If the
swr is say 2:1 then you can probably just rely on your internal atu.  If the
swr is greater than this there is another simple approach I would favour.
Purists should stop reading at this point because they will cringe.  What I
would recommend is that you move your feedpoint off centre.  By doing this
you can readily find a point that will give you a good match at around 50
ohms.

Keep in mind that when using a low and bent dipole you have already forfeit
your pursuit of a text book radiation pattern. (BTW I'm not sure it was
really worth pursuing anyway.)  The only matter of significance that
remains, is getting what power you have into the antenna.  When you've done
that it WILL radiate.

Good luck.

73

Bob 5B4AGN, P3F, ZC4ZM, G3ZEM
----- Original Message -----
From: "john lemay" <john.lemay at which.net>
To: <uk-contest at contesting.com>
Sent: Tuesday, February 18, 2003 12:34 PM
Subject: [UK-CONTEST] Inverted V help


> Hello all
>
> In one of my very rare forays below 50MHz, I decided to try an inverted V
> for 80m. It would use my modest Versatower for the central support, and
the
> ends would end up where-ever.
>
> I knew it would be a compromise :-
>
> I probably could not get enough height
> I probably could not get the right apex angle
> One end in particular would need to be close to the house and bent through
> 90 degrees
> The other end would be in trees.
>
> Even so, I thought that at least I would get an antenna that would be
within
> the limits of a typical internal ATU on my rig.
>
> Wrong !
>
> It received quite well, and nicely quiet compared to my HF6V. But the
match,
> checked out on the MFJ analyser was about 10 to 1. I have done the obvious
> tests, such as continuity of wire, continuity through balun etc.
>
> Today, I have taken down the 1:1 balun (Watson) and checked it out with a
> resistor across the terminals. Interesting: - Using a 50ohm resistor which
> is suitable for low frequency rf use, the match through the balun at
3.0MHz
> is about 2 to 1. It gets worse going lf, and improves to 1.2 to 1 by about
> 10MHz. Then not surprisingly above 40MHz the match gets lots worse.
>
> What I need help in deciding, is whether my abysmal match with something
> resembling an inverted V is due to too many antenna compromises or is it
due
> to a dodgy balun ? (Preferably without too many more sessions freezing in
> the garden, or hauling the tower up and down).
>
> Regards
>
> John G4ZTR
>
>
>
>
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