Joe, K8FC wrote:
"Although I agree that omega is a good way to match a physically short
tower I would not give up on the simpler gamma match just yet. The best
matching system for feeding a grounded element, unless we are very lucky
or very careful in planning the installation, is a simple gamma capacitor
and shunt feed conductor with the tap point adjusted to find 50 ohms
resistive."
----------
The problem with a short tower without heavy top loading is that you
can't get high enough with the tap point to reach 50 ohms resistive. It
will be lower than 50 ohms. Adding the "omega" capacitor has the effect
of making the tap point higher.
==========
"It is my belief that virtually any reasonable height tower can be gamma
matched with in some cases many trips up and down but it can be done."
----------
To save a lot of trips up and down the tower, modeling the structure
properly will get you very close to the ballpark. I've done that for
many hams over the years and have been quite accurate.
The shortest tower without any top loading whatsoever that can be gamma
matched is about 85 feet. Rohn 25 that high with the tap point at the
top with 36" shunt wire spacing will have a feedpoint Z of 50 + jX.
A tower shorter than that requires an omega match (unless you increase
the shunt wire spacing to rediculous values or decrease the size of the
shunt wire to very small diameters.
==========
"Obviously the variables that determine bandwidth (gamma wire diameter)
and tap point (~50 ohm ) must be dealt with."
----------
Increasing the diameter of the shunt wire will slightly increase the 2:1
SWR bandwidth, but it also makes the required 50-ohm tap point higher.
Vice-versa on both things is the wire is thinner.
Agreed that the tap point (~50 ohms) is of prime importance.
==========
"Every one of these installations in my experience has been different."
----------
Agreed. My own two towers (spaced 135', which I use as a shunt fed 2-el
array on both 160m and 80m) are close to identical with a TH6DXX at 75'
on one of them and a 402BA at 75' on the other. The shunt wire tap point
on the TH6DXX tower is at about 60' for 50 ohms resistive. The other
tower with less top loading (the 402BA) requires the tap point to be at
65' for 50 ohms resistive.
==========
"In addition, it is imperative that long before ANY matching attempts of
the vertical grounded tower that an EFFECTIVE ground and radial system
must be in place prior to constructing the match."
----------
I wouldn't say that it's imperative. For example. when I first shunt fed
my towers on 160m in 1976, each tower had six 125' radials under it. I
kept adding radials until I had 96 of them, each 125', under each tower
(~25,000' of wire!). Nothing whatsoever changed on the shunt feed
tuning.
==========
"I understand that omega matching can definitely be used on short towers
but is it worth the trouble especially with the voltages on the omega
caps when running power?"
----------
The RF voltages across the cap(s) is a function of the electrical size of
the structure. On a structure that is self-resonant (1/4-wave), the
voltage will be least (~225 RMS volts at 1000 watts). Also, the value of
the gamma cap will be greatest and the 2:1 SWR bandwidth will be
greatest.
As you deviate from a structure that is 1/4-wave resonant, The voltage
across the caps increases, the value of the gamma cap decreases and the
2:1 SWR BW decreases. Worst case is where the structure is 1/2-wave
resonant, where the RF voltage across the gamma cap is over 9000 volts
RMS at 1500 watts. A smaller structure that is, say, 1/8-wave resonant,
is not as bad as that.
==========
"At best, the omega match saves us a few trips up and down the tower
while searching for the precise gamma shorting strap position between the
gamma wire or cage and the tower. If we are already at the top of a tower
with the gamma tap point, and if at that point we find resistance after
the reactance is canceled is too high, an Omega will not help."
----------
If the tap point is at the top of the tower and the resistive component
is greater than 50 ohms, that means you must lower the tap point!!!!!
==========
" The omega can only match loads LESS than 50 ohms resistive by stepping
the resistance up! It can NOT step or transform antenna feed resistance
downwards when using capacitors, and neither the omega or gamma can match
capacitive antenna loads."
----------
I totally agree. However, it is a fact that the higher the tap point the
higher the R will be, and vice-versa, and you will never run into
capacitive reactance (unless the structure is significantly larger than
1/4-wave resonant where there will be more than one 50-ohm tap point --
one of them with inductive reactance and the other with capacitive
reactance).
==========
"Adjustment of resistance (not the reactance) requires adjusting the
diameter, spacing, or length of the gamma section."
----------
Exactly. Keep in mind (especially for towers less than 1/4-wave
electrically) that:
1) Decreasing the diameter of the shunt wire increases the R.
2) Increasing the shunt wire spacing increases the R.
3) Going higher with the tap point increases the R.
And the vice-versa for all three.
73, de Earl, K6SE
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