[Antennas] Building A Coil

[Terry Conboy]

At 04:18 PM 2008-08-24, Mark wrote:
>I have an idea on how to improve my mobile antenna that I am using 
>on a portable tripod. I need some help to make my idea come to life 
>however. My antenna is a 75 meter mobile and it presumably acts as 
>an end fed quarter wave. I am looking for a coil that I could mount 
>between the feed point of the antenna and the mount. It would need 
>the appropriate 3/8 threaded adapters at each end. This coil needs 
>to act as a quarter wave resonator. This will bring the antenna to a 
>total of a half wave. Does any one know if such a coil is made on 
>the market or if there is some one I could higher to build one like this?

Your mobile antenna doesn't look like a 1/4 wl antenna.  It looks 
like a ~1/10 wl antenna with a loading inductor to obtain 
resonance.  As long as it is worked against ground, you can't make it 
radiate like a 1/2 wl antenna (or even a 1/4 wl antenna) through any 
matching tricks.  It's still has a pattern like a 1/10 wl antenna, 
just with a different feedpoint Z.  Yes, you can modify the current 
distribution on the radiator by moving the loading coil up or adding 
a top hat, but that primarily impacts the efficiency, with minimal 
impact on the elevation pattern.

It should be possible to add enough center loading inductance 
(roughly double) to get a short floating vertical dipole to resonate 
without a connection to ground.  However, it's unlikely that it will 
be a much better radiator than a loaded monopole (such as a typical 
mobile antenna).  Of course, if you can't get a reasonably low loss 
ground connection, you may have no other choice.  Many configurations 
of the Buddipole employ this type of loading for a floating antenna.

It's important to realize that even with lossless coils, a loaded 
short vertical dipole won't have any more gain than a monopole of the 
same height.  For such an antenna to have gain, the overall physical 
size has to actually be near 1/2 wl.  It's much better to think of 
the loading coil as just supplying inductive reactance to resonate 
the inherent capacitive reactance of a short dipole/monopole.  Adding 
more coiled up wire than is needed for resonance doesn't really buy 
you anything except more loss resistance and more inductive reactance 
that you don't need (except for the small added inductance needed for 
matching with a shunt capacitance to provide a virtual L-network to 
step up the low impedance of a short antenna to 50 ohms, etc.)

73, Terry N6RY