[TowerTalk] Folded Monopole

[w8ji.tom]

Hi Larry,

 
> The article in the compedium is short, but my understanding of antenna
> 	theory would make me believe that the increase in the feedpoint
impedance
> 	would make the matching network less critical for losses.  The antenna
> 	also is noted to appear inductive, so that matching can be done with
> 	a parallel capacitor across the load to tune out the reactance, and fed
> 	through another series capacitor for matching.

Circulating currents and losses are a function of VAR (volt-amperes
reactive) power as well as the resistive part of the impedance. With an
omega match, losses are always higher than the same system with a direct
feed because VAR power and system operating Q is higher. The system still
has all the same currents in the radiator, the feed system just adds
additional transmission line type losses in the feed wires.  

Not that it matters, because those types of losses are so small they can be
ignored. The real killer is ground loss, and that doesn't change one bit.  


> 	What I find interesting is the shorting cable to also help conjugate
matching
> 	of the feedpoint impedance.

That reduces VAR power and losses, because system Q is lowered. While loss
reduction is insignificant, you'll get a lot more bandwidth.

> 	So, the question, now, the three vertical wires spaced approximately 30
inches around
> 	the tower will be 45 feet in length.  The tower is a 50 foot tower. 
This is still a
> 	short vertical for 160 meters, is the antenna in fact going to appear
inductive? '

Depends on what is on and near the tower. With a common size tribander it
will probably look a little short for 160. 

> 	I still going to need a low resistance hi Q coil between the feedpoint
and the ground
> 	system to resonate it? 

Probably not, because the feed wires will add series inductance all on
their own. The best thing to do is try it. Systems like that almost always
work very well, no matter what the exact "resonant frequency" of the tower
is.

All this stuff about matching systems (gamma match, omega match, etc)
affecting loss is overrated. Unless you match with a resistor you'll never
know the difference. Bandwidth, however, is greatly affected by the
matching system. A good shunt feed system (gamma match) with a thick feed
system (skirt) has more bandwidth than anything other than a series fed
system.

This is an example of a system where narrow bandwidth can mean more loss,
though the amount of loss  is insignificant.

73 Tom

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