[Elecraft] EFHW

[K9MA]

On 2/11/2017 13:03, Brian Pietrzyk wrote:
> Now comes the fantasy thinking... What about making it an end fed 
> multi band by either putting traps in the 80m end fed or better still 
> adding on a couple of fan EFHW wires for 40 and 20? Anyone tried this? 
> How would the efficiencies compare to their centre fed trap or fan 
> dipole counterparts respectively? 

That won't work, because the shorter wires will present a much lower 
impedance, and essentially short out the half wave wire.  The whole idea 
of the EFHW is that the high feedpoint impedance reduces ground system 
current to practically zero.

A trap might work, but might not be compatible with keeping the antenna 
very light.  A QRP trap could be pretty small, I suppose, but it still 
might be hard to support the wire with the extra weight on it using a 
lightweight pole.

However, an EFHW will work on harmonics, though the pattern changes.  A 
40 meter half wave works quite well on 20, but I'm not sure an 80 meter 
half wave would be all that great on 20, especially with a single 
support.  The free space patterns of long wire antennas are shown in the 
ARRL Antenna Book.  As frequency goes up, more lobes appear in the 
pattern, and the main ones get closer to the axis of the wire.  For 
example, a full wave antenna has a cloverleaf pattern, the lobes about 
55 degrees from the wire axis, with nulls broadside.  For a 2 wavelength 
wire, there are 8 lobes, the main ones 35 degrees from the wire axis.  
These patterns will, of course, be modified by the effects of ground and 
the fact that the wire probably won't be straight, but the free space 
patterns give some idea of how the antenna will radiate.  Modeling these 
antennas is very easy, and gives much more accurate results.

Note that the pattern of an EFHW (full wave, etc.) is NOT the same as 
that of a center fed dipole operated at even harmonics.

73,

Scott  K9MA


-- 
Scott  K9MA

k9ma at sdellington.us