On 7 jun, 23:50, "Nick" wrote:
I am wanting to experiment making vertical whips, end fed, for a number of
frequencies
from HF through to UHF and was wondering if someone can point me to a link
with
designs for constructing these.

I want to feed with 50 ohm co-ax for convenience, and I think it basically
has to
resonate at the design frequency, tapped for 50 ohms to feed it and present
a
"high impedance" to the end of the resonant element.

Naturally there will be different designs required for UHF frequencies
compared
to HF frequencies for mechanical considerations and I guess different
matching networks
with different Q's will have pros and cons too..

Anyone with some links / info would be greatly appreciated

Thanks

Nick

Hi nick,

When you want to be ground plane independent, a halve wave resonance
is the best option. It does introduce ground currents, but far less
with respect to 1/4 or 5/8 WL. Please note, that you still need some
ground, but a motor cycle frame will be sufficient. When you want to
have full benefit of the HW, you must assure that it resonates as a
half wave (so it will be shorter then an physical half wave). I would
recommend familiarizing yourself with an EM simulation program. It may
save you a lot of experimenting (with negative results).

While the center fed impedance of a HW dipole is not strongly
depending on thickness of radiator, the end fed impedance is. An HF
whip (5m for 10m) will be somewhat below 1 kOhm, however a UHF thick
whip (5mm/70cm band) over a small groundplane will be in the 500 Ohms
range.

The disadvantage of the HW end-fed is his length. When you want to
make it shorter (for example the coil or helix construction), its end-
fed impedance increases significantly. When you halve the length (but
keep halve wave resonance), you may expect a factor 2...4 increase in
impedance.

About matching.
Mechanically, end-fed matching is easiest, you can put the ground to
the motor cycle frame. Elevated feeding gives low feed impedance, but
you also have to include a common mode (balun) function.
For end-feed, you may use an LC section (L in series with coax, C
parallel to antenna), wound transformer, or combination of both.

I assume that you have some means to measure VWSR and or impedance and
don't touch the ends with there is relative high voltage....

Best regards,

Wim
PA3DJS