[TowerTalk] "Q" (was no-subject)

[Tom Rauch]

Hi Dave,

>  I am no expert in this field but by definition, low Q means high loss. Q
>  is 
> the
>  ratio of reactance to resistance in a parallel RLC network. A trap is
> essentially
>  a parallel RLC network. While low Q traps are better behaved and less
> finincky
>  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 
> tests
>  are run on an antenna with high Q traps and the same antenna with low Q
> traps,
>  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 
resistance value. 

>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 
antenna.  

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 
used. 

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 
performance goes.

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 
bandwidth. 

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 
improved. 

It's like deja-vue again looking at these traps.
73, Tom W8JI
w8ji@contesting.com

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