Topband: Fw: 160 m inverted L

Tue Nov 8 12:34:35 EST 2016

From: Charles Moizeau <w2sh at msn.com>
Sent: Tuesday, November 8, 2016 12:32 PM
To: farrerj at 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 at contesting.com> on behalf of John Farrer via Topband <topband at contesting.com>
Sent: Tuesday, November 8, 2016 2:43 AM
To: Wes Attaway (N5WA)
Cc: topband at 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 at 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 at contesting.com] On Behalf Of Art Heft
> Sent: Monday, November 07, 2016 3:06 PM
> To: topband at 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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