Cecil Moore wrote:
Roy Lewallen wrote:
1. Winding a half wavelength of wire doesn't make an antenna which
acts like a half wavelength antenna.

A helical antenna section can be treated as a piece
of transmission line with a velocity factor and a
characteristic impedance. Knowing the velocity factor
allows one to calculate the approximate length of
the section. On my web page, I have an EXCEL file
that will estimate the VF and Z0 of a helical
antenna: http://www.w5dxp.com/coilZ0VF.xls

To see how this might work, let's assume a 1/2WL
helical dipole for 40m using a helical of 2 inches
diameter and 2 turns per inch on 7.2 MHz. If the
value at A7 on the spreadsheet is less than one,
the VF and Z0 will be in the correct ballpark
- probably within 15%.

For such a coil, the VF is calculated to be 0.207.
1/2WL at 7.2 MHz is 68.3 feet. So how long should
the coil be to be 1/2WL long?

68.3 feet times 0.207 = ~14 feet

Of course, this is not an exact length and must
have turns subtracted (or added) to bring it to
resonance on 7.2 MHz. Since end effects are not
included, the dipole will probably resonate on
a frequency lower than 7.2 MHz. Removing turns
will then bring it to resonance.

Use half of the above for a 1/4WL vertical
section approximately 7 feet high.


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Thanks Roy and Cecil. I'll work on this using your
comments/suggestions.

I'm in a vy antenna unfriendly area: an apartment in the air shaft of
the bldg. So I'm abt 40' from free-air on all four sides. Fortunately,
I'm on the top floor, so I'm trying to get a "stealth" antenna to place
on the roof: something that would be easy to set up/take down. The
helical fit that description. But, I'm running QRP (abt 5W) and, taking
into consideration what Roy said abt power & efficiency, I'm probably
not getting anything out.

One good thing abt this antenna is, for it's size, it does receive
better than the random-length wire (abt a 50' rectangle) I had been
using inside my apartment. It does have good bandwith - but, as Roy
stated this indicates low radiation resistance.

Cecil, I'll work w/ your s/s and play around some more w/ the helical
(just for the fun of it, ya know ;-) ).

If this doesn't work out I'll try a linearly loaded dipole that N5ESE
has on his website.

Thanks to both for your advice.

73

--
MGFoster:::mgf00 at earthlink decimal-point net (KI6OFN)
Oakland, CA (USA)


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