I agree with the comments so far, and also that a 2:1 transmission line
transformer is a good method. But if you are in a hurry, here is an very easy
approach.
Just use a hairpin match. Mine was just some 1/8" copper tubing (but #8, 10 or
#12 wire would be fine too) wound around a wine bottle for about 6 to 8 turns.
Remove the bottle and reserve it to drink upon success! The coil goes from the
feedpoint to ground. (Yes, just like you are shorting the antenna. But you
really aren't.) Then just stretch it open or closed to get best SWR from an
analyzer.
In my case it brought my antenna from about 25 ohms up to about 45 ohms to give
a plenty good enough SWR. Please note that this method will also shift the
resonant frequency, so you may have to shift top wire length a bit as you
adjust the coil.
It took me half an hour start to finish to get a good match, no parts required.
Many 40M shortened yagis use this method.
73,
Drew K3PA
------------------------------
Message: 6
Date: Thu, 1 Dec 2016 16:53:53 -0800
From: Grant Saviers <grants2@pacbell.net>
To: james soto <kp2bh@yahoo.com>, Topband <topband@contesting.com>
Subject: Re: Topband: Inverted L Radials
Message-ID: <5840C621.6000403@pacbell.net>
Content-Type: text/plain; charset=utf-8; format=flowed
As you were disconnecting the radials the ground losses were increasing
until it got to enough ohms and you had a good match at 50ohms.
My 160 T matches 25 ohms 1.12:1 at 1812Khz and is fed with a 50:25
transmission line transformer. With 8x 125' elevated radials the N6LF
research shows I have fairly low ground losses.
As I added radials, I tracked my swr and it steadily decreased, pretty
much asymptotic to 25 ohms. Different L or T geometries will yield
different resonant impedances, but most will be well less than the full
1/4 wavelength 36 ohms. With an analyzer you could measure the Z as you
reconnect radials and learn a lot about your antenna SYSTEM.
TLT's are winners IMO for matching resonant antennas, wide bandwidths
,very low losses, and essentially no sensitivity to the environment.
Grant KZ1W
On 12/1/2016 12:14 PM, K1FZ-Bruce wrote:
> This is normal, but without the radials your RF is mostly heating up
> the earth.
> If you make the L a little longer, then insert a variable capacitor at
> the feed point, you should be able to find a higher feed point with
> the radials connected..
> With trial and error 'juggling' the extra wire length and capacitance.
> you can get very close to 50 ohms, and get a good signal out for DX.
> 73
> Bruce-k1fz
> http://www.qsl.net/k1fz/beverage_antenna.html
>
>
>
> On Thu, 1 Dec 2016 19:59:57 +0000 (UTC), james soto via Topband wrote:
>
> Hi allLast year i install an inverted L antenna for 160 with few
> radials different lenght.ohms was between 20 to 25 ohms and swr about
> 2.2 . This past weekend i was checking thethe antenna with the mfj
> analyser and i proceed to disconnect the radials from the groundrod
> and just leave the ground portion of the coax attach to the ground rod
> and the reading wasSWR 1.2 and 50 ohms. are this normal ?
> Thanks
> KP2BH / jimmy
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|