[TowerTalk] "Q" (was no-subject)
Wed, 20 Sep 2000 10:30:52 -0400
> I am no expert in this field but by definition, low Q means high loss. Q
> ratio of reactance to resistance in a parallel RLC network. A trap is
> a parallel RLC network. While low Q traps are better behaved and less
> than high Q traps, they are lossy and thus rob gain from the antenna.
Q can define many parameters, and be measured many ways. Low
Q sometimes can produce higher efficiency, but in this case you
are right on target. Low Q would mean more loss.
> While I have no data to back this up, I'll bet a dollar to a donut that
> if pattern
> are run on an antenna with high Q traps and the same antenna with low Q
> the high Q traps will result in higher gain (no brainer).
No doubt about it, as long as we are talking about trap Q.
What we need to do is translate the trap "Q" into an equivalent
>From the equivalent series resistance, and current though the trap,
we can get the dissipation in the trap (and loss) when the trap is in
a "pass mode". A few traps I've measured have 2 to 4 ohms
resistance in this mode.
Now before someone misapplies this information using the formula
eff = Rr+Rloss and says with a two ohm trap resistance and a 50
ohm Rr that is 96% efficiency....you have to normalize the traps
resistance to the feedpoint resistance considering the transmission
line effects of the antenna. The trap is inserted out near the end of
the element and so the impedance at that point is higher than the
Rr taken at the current maxima.
Two traps with 2 ohms ESR added about .02 dB loss to a dipole
At the isolation frequency, you have to consider the effective
parallel resistance of the trap.
It appears so far that is the larger effect.
It's early yet and I'm still gathering numbers, but so far it appears
the largest ill effect of traps is they greatly narrow the bandwidth of
the elements on bands above the lowest band where the antenna is
It appears the ill effects are mostly that of causing the antenna to
be tuning critical in the parasitic elements, rather than actually
dissipating power. What it seems like so far, although it is too
early to say for sure, is that trap antennas probably mistune the
parasitic elements to get bandwidth...and that's where most of the
If that is the case, the solution would be to use a single trapped
driven element and perhaps separate full size elements for
reflectors and directors....or some combination of traps and full
size parasitic elements.
So far, actual dissipative losses appear to be very low.
I worked on a trap antenna for Heathkit. I quickly learned no matter
how thick the element was, the trap is what set bandwidth on
bands above the lowest band. A trap greatly reduced the
Lowering Q to increase bandwidth resulted in unacceptable power
loss and very little bandwidth improvement, it was impossible to
keep from melting the traps at high power yet bandwidth barely
It's like deja-vue again looking at these traps.
73, Tom W8JI
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