Push-up Mast as a Low Band Vertical Antenna - de N4KG
Back in 1970 I used a 50 ft push-up mast with a 10 ft TV mast extension
as a support for an 80M inverted vee fed with open wire line. At the
top,
I added a short piece of wooden dowel to separate the feedpoint from
the metal support. The ladderline was pulled off to the side to prevent
contact with the mast and heavy duty plastic clothesline guys (no wire).
At the base, I used a heavy glass Coke bottle as an insulator. A simple
L-network was used to match the mast as a near half-wave vertical on 40M.
It was a KILLER for DX! I never tried feeding the mast as a vertical on
80M
since I had not put out any radials. Adding radials and a small loading
coil (with relay or aligator clip switching) to provide 80M coverage
would
seem to be a good idea.
On 20M, a 50 ft vertical acts a 3/4 wavelength radiator. While there is
a
major low angle lobe, most of the energy will be radiated in the major
lobes at 60 degrees above the horizon which will pass through the
ionosphere into space at 14 MHz. A 60 ft mast will have little low angle
radiation on 20M.
The difference between galvanized steel and aluminum should not
have any noticable effect since the ohmic losses are small compared
with the radiation resistance of the antenna.
de Tom N4KG
.............................
On Mon, 8 Dec 1997 00:56:02 -0800 (PST) ftbrady@earthlink.net (Frank T.
Brady) writes:
>TowerTalkians,
>
> I'm ready to begin experimenting with the telescoping mast/vertical I
>bought recently:
>
>* I want it to double as the support for a wire antenna between it and
>the
>same mast type about 140 feet away.
>
>* I'm erecting it above the leach lines of our septic system in the
>hope
>that it will cut down on return current losses.
>
>* I want to use it mainly on 40 (and 20 if possible) and I would like
>to try
>various physical and electrical radiator lengths (like maybe 5/8
>wavelength).
>
> I described the project to N5SU in Dallas. He has a 52' aluminum
>tower
>vertical that puts out a GREAT signal on 40 meters. He warned me to
>watch
>out for coupling between the wire antenna flattop and/or feedline and
>the
>vertical. He and others also said that galvanized steel isn't a good
>rf
>conductor, so I should add a bonding strap between each section.
>
>So my questions are:
>
>Will a galvanized steel mast be less efficient than an aluminum
>radiator?
>
>Are there vertical modeling programs that would help me pick the best
>length
>between 44' and, say, the max of 61 feet or so that I could attain
>with a
>whip attached to the top of the mast? The mast will be guyed at all
>sections and the whip would (hopefully) ride safely atop the mast.
>
>Can modeling determine whether or not coupling may occur and whether
>or not
>it would be detrimental or perhaps just be an influence on the length
>like a
>capacitive hat?
>
>Will the vertical interact/couple with the open wire line dropping
>vertically from the center of the wire antenna - a minimum of 70 feet
>away?
>I can separate them more if modeling information dictates.
>
>Although I'm only going to be running 100w for quite some time, If
>trying to
>load this on 20 considerably complicates things, I'd prefer to put 20
>off
>for awhile. In any case, optimum 40m operation is the primary goal.
>
>If any of you know of modeling programs or personal experience that
>can help
>me before I sink the concrete guy anchor pads for these two masts, I'd
>greatly appreciate it.
>
>73s and seasons greetings,
>Frank T. Brady - W0ECS
>Wildomar CA
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