[Harc] Antenna Elmer needed
MJ Inabnit
ke6sls at arrl.net
Sat Apr 25 12:51:39 CDT 2020
Hi Richard,
First off, BRAVO for doing your own builds om! Lots of fun and fairly
easy to build a decent antenna, especially for VHF and UHF since they
are so small!
So you are using a dipole. You could be doing MUCH better if you built
a vertical and it is still very easy. But, you are using a dipole to
achieve 0db gain (power in roughly equals power out minus the losses of
coax and connectors.)
The simple formula is 468/Freq in Megahertz. So our number is
468/146 which gives us 3.2 feet, or roughly 39 inches.
So, I would cut two wires, (or other element material) about 20 inches,
take a SWR measurement then begin trimming equal lengths from each side
until you reach your happy minimum SWR which should be very close to 1:1.5.
this type of antenna is very much like to common 1/4~ magnetic verticals
we use on our cars. Obviously, the 1/4~ vertical uses the roof as the
other half of the element.
I don't think I would accept the sellers theroy that your antenna is out
of tune, a 1 1/2 SWR is will under 2 and our rigs should happily deal
with that swr.
So you are on the right track if I read your post correctly.
Now, consider building a 3/4~ vertical so you will have some GAIN! You
simply cut the radiator to 3/4~ length and four 1/4~ ground radials.
Keep up the good work OM!
73
j
On 4/25/20 10:02 AM, Richard Kern KN6FXK via Harc wrote:
> From: Richard Kern via Harc <harc at humboldt-arc.org>
> To: HARC <harc at humboldt-arc.org>
> Cc: Richard Kern at reninet.com>
>
> I need some assistance with a 2-meter home made dipole.
>
> I returned by Btech dual band transceiver to the seller because it
> stopped transmitting. They've did the repair/replacement and sent it
> back, telling me I killed my first one with an antenna that was out of tune.
>
> According to my NanoVNA, it's under 1.5 swr throughout the 144-148 range
> in which I operate. I did one 10 second transmit on the 444, but it did
> still work after that, at least for a while.
>
> So:
>
> if my vna is accurate, the antenna should be fine on 2 meters.
>
> My concern is that when it is measured, it seems too short compared to
> the calculations. Is it normal for a dipole to be about 80% of 1/2
> wavelength? It's copper wire, probably 14 gauge. I was expecting
> 90-95% of simple wavelength calculation. I haven't found anything in
> the literature concerning how much to trim, other than "a little at a
> time until it's swr is minimum for your frequencies).
>
> I don't want to hook it back up if I'm operating on bad data (from vna).
>
> Do any of you have a home-made, confirmed in-tune 2-meter antenna? If
> so, what is it made of and what are the lengths of the elements?
>
> I welcome suggestions.
>
> Richard
> kn6fxk
>
>
--
wishing you well
Jaye, ke6sls--via the toshiba w/thunderchicken
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