Topband: Modelling/calculating the radiation efficiency of short verticals (correction)

[Terry Conboy]

At 05:25 PM 2006-10-12, George Stewart, VK5ALS wrote:
>Can anyone give me some help in arriving at the change in radiation
>resistance (efficiency) for a 40 foot vertical on 1.8MHz with a top capacity
>hat 20 feet in diameter with 8 radial arms that then has a 30 foot "whip"
>placed above the capacity hat?
>
>I do not know how to start to work out a problem like this one!
>
>Common sense would indicate that the shielding effect of such a capacity hat
>would largely negate the effect of the "whip".
>
>(The "whip" I have in mind is a fibre glass fishing rod blank with a copper
>wire run up the inside of the rod).
>
>I cannot help but think that going from say a 40 foot vertical to a 70 foot
>vertical must make quite a difference especially as the Achilles heel of the
>whole system is the inability to put in anything like a decent ground for it
>to work against.
>
>No doubt there is a combination of top hat diameter and 30 foot "whip" which
>would theoretically outperform the 40 foot vertical with a 20 foot diameter
>capacity hat?


You can get a fairly good idea how this works using EZNEC. - [If you 
aren't an idiot and in a hurry and make several errors like N6RY 
did!  I erroneously used a 20' radius hat, not a 20' diameter, plus I 
used the 3:1 SWR BW by mistake.  Here are some more reasonable numbers.]

I used 0.25" conductors everywhere with your dimensions.  Assumed a 5 
ohm ground loss (pretty good system) and used the "MININEC" ground in 
EZNEC (NEC-2 based) with "average ground" parameters.  I added series 
inductive base loading (Q=200) to obtain resonance at 1.83 
MHz.  Here's what I got:

hat and whip
Gain = -0.8 dBi at 25  degrees, BW = 47 kHz, Zin = 14.81 ohms, Zl = 
1.09 + j 218.6 ohms

hat, no whip
Gain = -1.69 dBi at 25 degrees, BW = 34 kHz, Zin = 12.8 ohms, Zl = 
1.67 + j 334.7 ohms

whip, no hat
Gain = -1.3 dBi at 25 degrees, BW = 32 kHz, Zin = 15.28 ohms, Zl = 
2.25 + 449.9 ohms

The radiation resistance is approximately Zin minus the 5 ohms ground 
loss and the resistance of the base loading coil.  The BW is between 
2:1 SWR points and is referenced to Zin.

It would appear that adding any size hat and/or whip would improve 
the performance, but the question is whether the work and cost are 
worth the small gain.

73, Terry N6RY