Topband: Inverted L

[Rik van Riel]

On 11/29/2011 09:56 PM, Carol Richards wrote:
> I have a few questions which I am hopeful that the gurus can help me with.
>
> 1. Using an rf-choke at the feed point...How many turns and what diameter?

No.

160m is a very wide band, 200kHz at 2MHz is about 10%.
This results in your wire antenna being quite reactive
on a large part of the band.

On one part of the band, the antenna will be inductive,
which is just fine with an inductive choke.

On the other side of the band, the antenna will be
capacitive.  The inductance of a coil wound choke can
cancel out some of the capacitance, resulting in MORE
RF on the outside of the coax than you would have
without a choke!

G3TXQ has good info on what kind of cores and how many
windings to use to create proper resistive chokes, on
ferrite cores.  The page also has a more detailed
explanation on why reactive chokes can be bad:

http://www.karinya.net/g3txq/chokes/

Chances are that on 160m, a choke on a ferrite core is
going to be cheaper than an entirely coax wound one,
anyway.

> 2. Where can I get 200-500feet of 75 ohm tv cable cheaply?

Many places online sell RG6 cheaply.  I have seen 1000ft
of quad shield RG6 go for as little as $100.

> 3. Vertical element about 100 feet, 3-4ft from the tower and horizontal component 35 feet 3 away from tower....I was told to use elevated radials tuned in pairs. I tried tuning each pair as dipoles, starting at 135 feet, each half and cutting off about 10 feet from each. Never really saw a dip on the mfj analyzer. SWR in the shack is 1:3. I think my 135 ft L might be too long......Question: Am I in the ball park with what I have done? I did work KH6LC and VK6HD and several zone 3's in CQWWCW easilyand had no problem breaking the pile-ups to the carribean, but it just doesn't seem to play well to EU. Of course it could have been a high angle night when I tried.Any thoughts?

Your MFJ analyzer also goes outside of the 160m band.
Do you see any dips below or above the 160m frequencies?

I measured the SWR on 5 frequencies (1800, 1850, 1900,
1950, 2000) while trimming my 160m antenna.  The goal
was to get a reasonable SWR on the lower half of the band,
while still getting something my ATU can handle on the
top part of the band.

Does the 3:1 SWR happen with an impedance that is lower
or higher than 50 ohms?

How is the SWR at various points in the band?

Again, 160m wire antennas can be quite narrow band
compared to the width of the band.  Having some
compromise length that allows your antenna tuner to
cover most of the band may be perfectly fine.

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