The business of avoiding high angle radiation at all costs is a real urban
myth on 160. NVIS closing of close-in skip zones on 160 can save your run
frequency in contests.
While creating an R=50, X= whatever feed Z is certainly useful, other
issues are paramount. Reduction of current in the counterpoise is always a
reduction in ground losses. For situations where the loss is unavoidably
high, going beyond R=50 may be worth it for efficiency reasons.
There is also the significantly erroneous myth that high angle radiation
from an L is taken away from the low angles. Some time in modeling shows
that higher efficiency provides the difference. The low angle radiation
remains fairly constant and simply depends on the height of the vertical
wire and the shape of the RF current density on the vertical wire.
There is also the myth that high angle makes RX worse. Being plain jane
vertical in part is what makes RX worse. To that end, to really hear on
160, one uses RX antennas to hear. In that case, who cares to what degree a
TX antenna hears. You can't fix the TX antenna's problem that it is a large
vertical antenna that will hear all the local vertically polarized noise.
The fix is to use various RX antennas. So who cares about TX antennas' RX.
If you are serious, you will get RX antennas. Many different RX antenna
solutions, workable, effective, beats RX-on-TX-antenna to a bloody pulp.
Then there is the business of a long horizontal wire moving the current
density up to fill out the entire vertical run, and getting more RF out at
low angles over the local RF-lossy ground clutter.
It is convenient that an emerging efficient range of an L's Hor + Vert wire
= 140-155 feet, does conveniently and decently meet other goals.
If you are doing QRP or 100W in a contest, filling in the skip zones caused
by pure vertical antennas will help you do running instead of all S&P, and
significantly improve your score, and yes, I meant running instead of all
S&P while QRP on 160m.
73, Guy K2AV
On Tue, Nov 8, 2016 at 12:34 PM, Charles Moizeau <w2sh@msn.com> wrote:
> From: Charles Moizeau <w2sh@msn.com>
> Sent: Tuesday, November 8, 2016 12:32 PM
> To: farrerj@yahoo.com
> Subject: Re: Topband: 160 m inverted L
>
>
> An advantage of having the total length of the inverted L antenna at
> greater than 0.25 wavelength is that matching to coax at its base can be
> achieved with just a capacitor in series between the center conductor and
> the antenna. No lossy inductors are present. Also, a costly vacuum
> variable capacitor overcomes losses in the capacitor rotor's thrust
> bearing. (The same benefit can be more cheaply obtained by bridging the
> thrust bearing with a piece of braided solder wick, or using a split-stator
> or butterfly capacitor with its rotor left floating, though finding such a
> capacitor with enough capacitance for 160m can be difficult).
>
>
> Assuming 128 feet to be the equivalent of 0.25 wl on 160m, John's
> arrangement puts the current maximum at 128 point back from the L's open
> end. This is true for all antennas having a total length greater than 0.25
> wl. And in John's case the current maximum occurs at a point that is 17
> feet up from its base.
>
>
> For low-angle radiation, arranging the L's total length to concentrate a
> maximum of current in the vertical leg requires the maximum current point
> to be situated at the midpoint of the vertical leg. Look at figure 2.5 in
> Moxon's worthy book to see this nicely illustrated. In John's case a
> horizontal leg of 95.5 feet would achieve this.
>
>
> However, there are disadvantages of such an arrangement.
>
>
> First, a longer horizontal leg will attract more high-angle signals while
> receiving. This can be eliminated by making the antenna a vertical Tee
> with two legs, each 95.5 feet long and faced 180 degrees apart, and this
> will also cancel high-angle transmitted radiation.
>
>
> Second, if you accept the premise that the function of radials, in this
> case assuming radials lain on the ground, is to collect radiation from the
> vertical leg that splashes off the ground and return it to the feedpoint
> for "recycling", then moving the antenna's maximum current point up to the
> vertical leg's midpoint will have the radiation splashing further away from
> the antenna base than would occur with a maximum current point at 17 feet
> high, as with John's present antenna. Obviously the higher in the vertical
> leg the maximum current occurs, then longer radials will be required for
> greater collection effect. Even at a 17-foot height radials should be
> longer than they would be with a total antenna length of 128 feet, with its
> maximum current point at ground zero.
>
>
> 73,
>
>
> Charles, W2SH
>
>
>
> From: Topband <topband-bounces@contesting.com> on behalf of John Farrer
> via Topband <topband@contesting.com>
> Sent: Tuesday, November 8, 2016 2:43 AM
> To: Wes Attaway (N5WA)
> Cc: topband@contesting.com; Art Heft
> Subject: Re: Topband: 160 m inverted L
>
> FWIW mine is cut by trial and error to 1825 and is approximately 65 feet
> vertical and 80 feet horizontal. I have moved it around to three different
> locations over the last 2 years. The dimensions change very little, perhaps
> 2 feet. The SWR can be tweaked by raising or lowering the FCP a little.
>
> Good luck
> John G3XHZ
>
>
> Sent from my iPhone
>
> > On 8 Nov 2016, at 03:23, Wes Attaway (N5WA) <wesattaway@bellsouth.net>
> wrote:
> >
> > I haven't run any numbers but it seems like you should just shorten the
> > horizontal wire (resonance freq is too low). I would go back to about
> 65'
> > horizontal. Somewhere in the range of 60' to 70' you should get close
> > enough.
> >
> > -------------------
> > Wes Attaway (N5WA)
> > (318) 393-3289 - Shreveport, LA
> > Computer/Cellphone Forensics
> > AttawayForensics.com
> > -------------------
> >
> > -----Original Message-----
> > From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Art
> Heft
> > Sent: Monday, November 07, 2016 3:06 PM
> > To: topband@contesting.com
> > Subject: Topband: 160 m inverted L
> >
> > I finally got the inverted L up this afternoon. Vertical dimension is
> 65'
> > and the almost horizontal dimension is 95'. I am using a very carefully
> > built FCP and the commercial transformer. My SARK 110 shows resonance at
> > about 1.68 MHz but the resistive part is up around 1000 ohms. Taken
> right
> > at the antenna. Doesn't seem right to me. Any ideas?
> > 73, Art K8CIT Hillman MI
> > _________________
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> >
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