[TowerTalk] Traps

[Tom Rauch]

I'm not sure this post will make it through, but here are some 
points we should remember.

1.) Trap loss will **always** result in trap heating. While high loss 
won't necessarily show up with 100 watts, it will almost certainly 
show up with 1500 watts.

2.) Trap loss can't be "1 dB per trap" as a rule-of-thumb. The loss 
will vary with trap design, what element the trap is in. For example, 
in a three element antenna with optimum current, the driven 
element traps should have about two to four times the loss of 
parasitic element traps. On bands below cutoff of the trap, the 
traps closest to the boom will have the highest loss. On bands 
above cutoff frequency of a trap, the isolating trap will have the bulk 
of the loss.

I certainly hope a false rumor like "1 dB per trap" doesn't get 
started. That rumor on par with the rumor we've all heard that 
connectors have a loss of .1dB per connector. These rumors are 
harmful to people, because they cause them to make poor 
decisions...and once they get started they are almost impossible 
to stop.

Keep in mind we went through a similar thing when linear-loading 
started. Now we suffer using less efficient antennas because it is 
difficult to find a well-designed lumped loading antenna.

Now I can't say if some trapped antennas are as bad as measured 
or not, and the problem is most other people can't say that either. 
Without facts from multiple sources, all that happens is a peeing 
contest. So I'll measure some traps, pass them along (I hope) to a 
totally independent source who will also measure them, and then 
we will all know with excellent accuracy how much equivalent 
resistance and reactance the trap has at various frequencies.

A few simple 5 minute measurements of each trap will let anyone 
model the trap in an antenna, and we'll know within a few percent 
how much loss is accountable for in the trap and which trap has 
how much loss. That will be closer than you can rely on a Bird 
meter, and many times better than we can do in any field test of 
field strength or heat.
73, Tom W8JI
w8ji@contesting.com

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