[TowerTalk] PAN CAKE ANTENNA -TAK-ANTENNA

[Dan Zimmerman N3OX]

>  What is the theory behind the antenna configuration?

End-loaded super short dipole ... with the coils simultaneously acting
as loading inductors and capacitance hats, with unknown efficiency.

>  Has anyone attempted to model the antenna?

Yeah, I just did an antenna that's got spiral hats, 3 feet long, that
happened to resonate in the 17m band.

http://www.n3ox.net/files/eznec/N3OX_spiral.ez

>  What is the expected input impedance at resonance?

For my antenna, about 7 ohms, which can be stepped up to 50 ohms with
a small coil across the feedpoint (might even be in the model
already).

>  Is there a difference between actual and advertised SWR 2:1 bandwidth?

I think the advertised SWR bandwidth is measured.  It is much, much
wider than that of my antenna (which is a noticeable bit longer in
wavelengths and has a higher radiation resistance than the TakTenna at
least.)

The 2:1 SWR bandwidth of my model is about 30kHz and the efficiency is
something in the 30% to 40% range I think.  It's an absolute necessity
to trade off bandwidth and efficiency, so the wider bandwidth antennas
of the same design and size are *guaranteed* to be less efficient by
quite a bit.

There's a curious thing about the spiral hats.  They seem to show
transmission line behavior (there is a current minimum and a phase
reversal somewhere about 2/3 the way out the spiral).  I think that
they may be acting as rather lossy transmission line stubs which could
be used to widen the bandwidth vs., say, the best possible end loaded
short dipole.
>
>
>  Has anyone confirmed SWR bandwidth claimed versus actual?
>

I'm not buying one to find out.  Might build my version someday, but
it's not nearly as wide-bandwidth.

>
>  Is there a defined radiation pattern or is it just a BLOB pattern?

My model shows a fairly well defined figure 8, really.

http://www.n3ox.net/files/N3OX_spiral.jpg

Just remember, the bandwidth vs. size vs. efficiency tradeoff is
fundamental.  There's no way around it, and don't believe anyone who
says there is some magic type of loading that fixes that.  It's all
about current flowing over an extended line.  If you make the line
shorter, you have to make the current go up.  If you make the current
go up, you either get more I^2*R losses in the conductors or you have
to reduce the loss resistance.  At the same time the radiation
resistance is going down, so the total resistive part of the feed
impedance is going down.  That means the Q goes up and the bandwidth
gets narrow.

No way around it.  Short antennas with wide bandwidths have them
because of loss.

73,
Dan