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