Interesting question. I have never reasearched this one, even though I
have thought about doing it, so I tried a few quick simulations. At
first the suggestion of making the reflector 5% longer seem to make
sense, but ... read on...
Using an 80 meter dipole, 133 ft long, at various heights, with a
reflector at 1 ft above ground with various lengths.
In the table below, first column is the reflector length, then seperate
columns for different driven element heights, gain figures are for
straight up (zenith).
R length 20ft 30ft 40ft 50ft
76 4.79 6.17 6.69 6.77
73 4.94 6.24 6.73 6.78
70 5.15 6.34 6.78 6.8
66 5.6 6.54 6.88 6.84
63 5.77 6.67 6.96 6.87
60 3.51 5.93 6.66 6.84
0 3.67 5.61 6.4 6.59
At first it may seem surprising that the peak gain seems to be for a
reflector that is a little shorter than the driven element. This is
because the reflector is being tuned to a lower resonant frequency by
the ground capacitance (which is higher than that for the driven
element). Actually, my simulations show the resonant frequency of this
132ft wire, 1 ft off ground is 2.6 MHz.
These numbers haven't been checked against any other references. It
represents a quick simulation done with EZNEC, using Real/High Accuracy
Ground, Cond = .005 s/m, diel const = 13. Since we are dealing with a
wire only 1 ft above ground, I would expect slightly different numbers
to be produced by NEC4, but the trend should be the same.
In practice, these simulations are not going to be accurate because of
the variations in ground parameters. You probably couldn't even measure
the resonant frequency of your ground reflector and keep it constant
because of varying conditions, such as rain. I simulated only one
reflector, but if you build this, you will want more than one.
If you are not interested in DX and only in generating maximum straight
up signal, the max gain appears around 55ft height for a 3.6MHz antenna.
Jerry, K4SAV
David Jordan wrote:
>Mark,
>
>If you are using a 1/2 wave dipole the wire on the ground should be
>unbroken and 5% longer. You can run two or three parallel wires spaced
>a few feet apart or just one wire.
>
>If you are using a full wave dipole then the reflector should be broken
>in the center. You're basically feeding two half wave antennas and the
>reflectors should be independent.
>
>My 2 Cents,
>dave
>wa3gin
>
>Mark . wrote:
>
>
>
>>Hi All,
>>All this talk lately of NVIS antenna work goes along with some antennas and
>>masts I have been building lately to enjoy casual, portable hamming, with
>>some emphasis on cheap antenna plans for a cheap emergency 80 trunk antenna.
>>So far, results have been quite good using 18ga lamp cord or speaker cord,
>>using a 30' or less of the unzipped portion as a feedline, to a 4:1 current
>>balun and then connected to the mobile HF rig.
>>
>>I once read that an old trick for boosting the efficiency of horizontal
>>antennas over questionalble ground was to run a series of radials along the
>>ground parallel to the antenna.
>>
>>For instance, if the antenna is a flat-top or inverted Vee, run 4-5 radials
>>parallel to it along the ground, spaced 10-15 few feet apart.
>>
>>This is pretty easy to accomply by unrolling precut wires from a homemade
>>spool (you can get free plastic spools by asking for the empty ones at the
>>home stores from their rotary wire supply racks).
>>
>>What is an effective placement for such wires under the antennas? Any rules
>>of thumb for length and orientation?
>>
>>If separate wires, do they need to be approximately 5% longer than the
>>driven antenna to act as reflectors?
>>
>>It would be convenient to just have a long wire on a spool and lay out a
>>zig-zag pattern around temporary stakes, such that you could just walk
>>around and pay out the line from the spools. Winding them up would be a
>>snap, too.
>>
>>Thanks for your ideas.
>>
>>--...MARK_N1LO...--
>>
>>
>>_______________________________________________
>>
>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>>questions and ask for Sherman, W2FLA.
>>
>>_______________________________________________
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>>TowerTalk@contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>>
>>
>>
>>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
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