On 28 ago, 03:55, VK3XL wrote:
Hi all, do you remember the old CD Stationmaster antenna? It was a end
fed 1/2 wave. I would like to make one and I believe there are plans
around to describe how its done, but so far I have not been able to
locate them on the net. Does anyone know of a link to such a page, or
even a magazine article etc. My work Library is quite good with regard
to old ham magasines etc.

Any assistance is greatly appreciated.
thanks for looking
Mike
VK3XL

--
VK3XL

High Mike,

The stationmaster is a HW end-fed antenna with LC matching section (L
between center of coax and radiator, C is capacitance between radiator
and radiator support construction).

You can make such an antenna for other frequencies (I did it recently
for VHF air band). The advantage of the HW over de 1/4W antenna is the
feed point impedance. Depending on the length/diameter ratio of the
radiator, the end-fed impedance varies between about 300 Ohm and 3
kOhms. When the radiator is horizontally, close over a reasonable
conducting ground, end-fed impedance may rise up to 10 kOhms. This
will result in High Voltage at the feed point as well.

The ground current (or coax screen current when no ground construction
is present), is about a factor sqrt(Zend-fed/40) lower with respect to
a 1/4W antenna (can be a factor 8). As long as the impedance that is
used as ground (or counterpoise) is far below the end-fed impedance,
the antenna works well. Gain and radiation pattern will be same as for
HW dipole.

There are some things that can go wrong. Most people tune by changing
the length of the antenna. That may result in the radiator to work
off-resonance, reducing the end-fed impedance and increasing the feed
current for the counterpoise. So in my opinion best thing is first
tune the antenna (without matching network and low power) to minimum
counterpoise current (measure with current probe) and then do the
matching network.

For the LC matching network, the C can be the capacitance between the
radiator and the metallic structure that supports the antenna. I
prefer the LC network above the LC tank circuit because of the higher
Q-factor of the tank circuit. This reduces the useful bandwidth of the
antenna significantly. As the matching is under the radiator, tuning
is not that easy.

When you need further information, don't hesitate to repost.

Best regards,

Wim
PA3DJS
www.tetech.nl