I would go further and say don't even bother with a
tuner for a 2:1 SWR. Any PI network in an amplifier
is capable of handling that 2:1 SWR. Just tune for
maximum power transfer (Output).
I would also use a T and not an inverted L, with
the exception of local stations within 300 miles all
others need the vertical approach and you don't want
to be wasting power in the Horizontal half of your
antenna. 73 Clive GM3POI
----- Original Message -----
From: "mrtman777" <mrtman777@bellsouth.net>
To: "topband topband" <topband@contesting.com>
Sent: Thursday, September 23, 2010 5:59 PM
Subject: Re: Topband: Inverted L Dimensions
Scott KB0FHP writes:
What dimensions are best - 5/16 or 1/4 wave? Both are
indicated in the
literature.
What is simpler to match?
George K8GG writes:
The easiest method to get 50 ohms is to make a 1/4 wl
Inverted-L,
and ignore the SWR at the feedpoint, feed it directly
with good quality 50
ohm coaxial cable into the shack and then use an
L-network antenna tuner
to match the impedance at the shack end of the
coaxial cable to 50
ohms. TenTec model 229 or 238
antenna tuners, or some of the military surplus
L-network antenna tuners
work very well for this application.
There will be very little loss from SWR in the
coaxial cable at 1.8
MHz.
George, I agree with you completely. First, let me
add a supporting argument
for the 1/4 wl inverted L.
Since the inverted-L will require a matching network
whether it is 5/16 or 1/4
wl, that leaves us using other factors to decide
which length to use. To
increase the DX performance of the antenna, you want
to maximize the low angle
radiation. We know that the current in an antenna
element is what causes
radiation, not the voltage. With the 1/4 wl antenna,
you get the maximum
current at the feedpoint. Obviously, and as is
indicated in the literature,
this typical 1/4 wl inverted-L will have alot more
radiation coming from the
vertical portion than coming from the horizontal
portion, as the highest
percentage of current is located there. By increasing
the length of the
inverted L to 5/16 wl, you move the current maxima up
the antenna. If you kept
lengthening the antenna, you would eventually get the
current maxima to
the junction of the vertical and horizontal parts. In
this case the
horizontal part would have as much current and
radiate as much as the vertical
part. So by lengthening the antenna over 1/4 wl, you
decrease the percentage of
the total antenna current that is flowing in the
vertical portion from the 1/4
wl case. This isn't what we want. We want maximum
current flowing in
the vertical portion for improved DX performance. By
using a 1/4 wave
inverted-L over a 5/16 wave inverted-L, you gain some
low angle radiation for
that hard DX path, at the expense of a bit of high
angle radiation for
stateside, where the signals are always way stronger.
This few extra dB can
serve you much better in working DX than for working
stateside. My 1/4 wave
still does fine stateside, and performed well in ARRL
160 last winter.
Second, my reasoning for matching at the shack as
George suggests instead of
matching at the antenna:
Cons:
Say for a long 1000ft. run of RG-213, you will only
lose an additional .35 dB
for a 2:1 SWR at 1.8 Mhz. I don't imagine that your
run will be nearly this
long. This kind of loss is neglible.
Pros:
You don't have to bother weather proofing your
matching network.
After laying radials all day, when you are ready to
tune up the antenna and try
it out, sitting there in your ham shack tuning the
network sure beats being out
at the antenna feedpoint on the ground doing it.
(Don't ask me how I know
this... hihi)
If you have problems this winter with the network, do
you want to be out in the
cold troubleshooting it, or in your warm shack
sitting down with a cup of coffee
at your workbench?
I believe all this is worth losing an extra fraction
of a dB in the coax.
The attached picture shows the network I use. Its
values were calculated with
ARRL's TLW (Transmission Line for Windows) that came
with the Antenna Book,
and the coax length was taken into account. I
determined the exact values with
a variable capacitor and a clip lead jumper on the
inductor and replaced them
with set-value components to save space. All of these
parts are out of my
junkbox, and it was built in an afternoon. Ignore the
relay. It is simply a
bypass switch for the network, so I can take it out
when using my L on other
bands (it is trapped). Note: The inductor stock is
homebrew patterned after a
recent article in QST by Barry Shackleford, W6YE.
73 and GL,
Trey KJ4FDV
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