[TowerTalk] PAN CAKE ANTENNA -TAK-ANTENNA

[David Gilbert]

That's pretty much been my impression of this antenna.  It generally 
gets rave reviews on eHam.net (doesn't everything?), but all you have to 
do is look at it to see a few obvious things:

1.  Three feet of linear element is going to have a VERY low radiation 
resistance.
2.  The spiral ends equate to lots of inductance and lots of distributed 
capacitance, none of which radiates effectively.
3.  Direct coax feed without a balun or shield choke is a dead give-away 
for dependency upon feedline radiation.

It's merely an expensive tank circuit at the end of a radiating 
feedline.  It would be interesting to see someone who owns one replace 
it with a traditional coil and capacitor and compare the performance, or 
as you say simply test it with and without a good common mode choke.

Dave  AB7E


K4SAV wrote:
> I recently got some real dimensions and rebuilt my model for this 
> antenna.  This antenna is supposed to cover 40 meters thru 10 meters by 
> selecting taps on the coils.
>
> Radiation resistance of the antenna on 40 meters without the feedline is 
> about 0.23 ohms.  Feedpoint impedance on 40 meters in free space without 
> a feedline or any kind of matching network is about 3.2 ohms.
>
> When you attach the feedline things change a lot.  With the proper taps 
> on the coils and with the right length feedline you can get a very broad 
> low SWR curve across the band (40 meters).  With the antenna at 20 feet, 
> the gain is about -2.4 dBi with a take-off angle of 30 degrees.  The 
> pattern is all vertically polarized (all feedline radiation).  Notice 
> that this antenna has no common mode choke and is fed directly with 
> coax.  When adding a good choke to remove the feedline radiation, the 
> gain goes to -9.5 dBi with a take-off angle of 90 degrees.  Some of this 
> gain reduction is due to feedline loss because removing the feedline 
> radiation also increases the SWR.  In this case since I only had 40 feet 
> of RG8X the coax loss was only 1.7 dB.  If the feedline had been 100 ft, 
> the gain would have gone to -12 dBi or 4.2 dB of coax loss.
>
> I expected to see the antenna do a little radiating on the higher bands, 
> but some unusual things happened.  Depending on feedline length and the 
> taps you select, you can direct a lot of current down the feedline or 
> not. In some cases with a lot of current down the feedline the take-off 
> angle was very high and low angle gain was low. In other cases with 
> different feedline lengths, the gain at low angles was reasonable (-2 to 
> -4 dBi).  You can't determine which condition you have by looking at the 
> SWR.  Playing with various combinations on 15 meters, the best gain I 
> ever got was -2 dBi and the worst was about -11 dBi.
>
> Summary: On 40 meters, the antenna acts as a variable unbalanced load 
> that injects large common mode currents onto the feedline.  The feedline 
> is the major radiator.  The thing on the top called the "antenna" is 
> used to fine tune the resonant frequency and SWR.  On the higher bands, 
> performance will be very unpredictable and can vary widely with each 
> installation.  In some cases it may approach the performance of a 
> no-radial vertical.
>
> Or so says EZNEC.
>
> Jerry, K4SAV