Topband: skirt-feed vs short gamma feed

[Herbert Schoenbohm]

Gary Nichols wrote:
> With the new receive system the tower is destroying the pattern especially when looking
> NE or SE but it still seems to hear pretty well in the NW and SW directions
> looking away from the tower. 
>
>  
>
>  After talking to Goose, W8AV I am following his advice to install a 3 drop
> wire de-tuning system to eliminate the interaction between the tower and RX
> system.  My question for the group is:  If I choose to use the de-tuning
> skirt for a transmit feed system instead of the short gamma rod system can I
> expect to see any improvement in my transmit signal? 
Gary,

The advantage of a three or four wire cage is greater bandwidth, a less 
reactive feed,  and as Goose mentioned the ability to open up the feed 
point to the bottom of the cage and have it act as an anti-resonant 
decoupling sleeve in respect to the tower.  This all depends on the cage 
size, tap point, and how close to a TB resonant frequency the tower is 
with or without the cage, open,  grounded, or fed via a matching unit. I 
think it would be better to model  the various cage connections. 
Depending on the dimensions, opening up a cage for 160 could give you an 
1/2 wave coaxial for 80 meters that could be a instant creation of a 
major noise source on 80 meters. It could also make a fine TX antenna 
for 80 with another shunt feed above the cage but in theory   give you 
about 3.2 db over the 1/4 wave and lower the TAO lobe about 10 degrees 
which could be good or bad. Yet I have read so many accounts that claim 
a 1/2 wave vertical on TB and 80  are not much of a benefit.

To try and answer your question, no, there is not any noticeable 
efficiency improvement with a cage feed according to the the NAB 
presentations I have read.  There is better B/W and an ability to 
control re-radiation via the anti-resonant decoupling sleeve and feed 
concept.  How much?  I'm not sure and don't think the modeling will be 
much help there except to reflect the highest reactance which I presume 
equates to less re-radiation.

But also remember that even steel tower guys that are broken up at 1/8 
wave intervals can contribute some noise to an exotic RX system close 
by.  There are a number of IEEE and NAB presentations on detuning large 
metal power line supports in the vicinity of multi-tower directional AM 
stations that are required to prove there patterns to the FCC.

73

Herb Schoenbohm, KV4FZ