[TowerTalk] Traps--Yagi Joint Losses

[K7GCO@aol.com]

In a message dated 7/18/00 1:22:21 AM Pacific Daylight Time, 
w8ji@contesting.com writes:

<< 
 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
  >>
An interesting test on a trapless yagi that has been up for some time is to 
give it a 5-10 minute 1KW or more carrier test and feel the first 2 joints of 
the Driven Element.  Test equipment needed is "one hand."  I've made 
believers out of Doubting Thomas's about the need for regular "Yagi Joint 
Maintenance."   K7GCO

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