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Re: Topband: Inverted L Dimensions

To: "topband topband" <>
Subject: Re: Topband: Inverted L Dimensions
From: "Clive GM3POI" <>
Date: Fri, 24 Sep 2010 09:20:11 -0000
List-post: <">>
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" <>
To: "topband topband" <>
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

What is simpler to match?

George K8GG writes:

The easiest method to get 50 ohms is to make a 1/4 wl 
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

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:

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.

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 

UR RST IS ... ... ..9 QSB QSB - hw? BK

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