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yagi query	Reply
by N4ZYV on September 21, 2000	Mail this to a friend!
Is there any advantage to insulating the driven element in a yagi from the boom, or is there an advantage to NOT insulating it from the boom? Any advantage to insulating the boom from the tower if the driven element is NOT insulated from the boom? Also I am going to build a 2 meter quad and wonder what effect an aluminum boom would have. If I use aluminum for the boom should I insulate it from the tower? How about using 2 reflectors for better front-back? Also, what is the unladen air speed of a sparrow?

RE: yagi query	Reply
by WB6BYU on September 22, 2000	Mail this to a friend!
Insulating the driven element of a yagi allows you to use beta matching, while if the center of the driven element is grounded you have to use a shunt-feedling system (T, gamma, or delta match) which may be more difficult to adjust for multi-band operation. Mechanically, grounding the element usually makes a simpler and more rugged construction... unless, of course, you are using a non-conducting boom. Grounding everything through the tower may have a slight advantage for lightening protection (but don't count on it). However, it will reduce the static build-up on the elements. Again, mechanically it is simpler to clamp the boom to the mast without insulators. A metal boom for a quad probably won't cause problems, but I have heard reports of problems if too much metal is used in the spreaders. By the way, there is an excellent article on quad design in the ARRL Antenna Compendium, volume I or II. It includes notes that using an odd number of directors is better than an even number, and I think he suggested using two reflectors on a 6 element quad as preferable to one (because of the resulting odd/even number of directors.) However, if the purpose of a reflector is to reflect the radiation, there will be relatively little reaching the second reflector, so one would expect it's contribution to be small. Do you want the answer in furlongs per fortnight, or angstroms per picosecond? - Dale WB6BYU

RE: yagi query	Reply
by K0RS on October 4, 2000	Mail this to a friend!
I think there is a significant advantage to having ALL parts of a Yagi grounded...static buildup. If there is a DC path to ground thru your tower, that can prevent damage to delicate components in your radio. It doesn't take a lightning strike to damage modern transistor circuitry. A relatively small static discharge will do the deed. You may also notice that your Yagi is quieter on RX when everything is grounded, especially during rain or snow storms. Lightning is so unpredictable that it is impossible to say that an antenna with all parts at ground potential would prevent any damage from a near strike, but it sure couldn't hurt. A direct strike will likely induce damage in any case. The best scenario is to have the tower WELL grounded to discourage a strike to begin with. A well grounded tower bleeds off the electrical potentential between the sky and ground before an arc (strike) occurs. The better the ground, the better the resistance to a strike.

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