Multiband antenna question - TowerTalk Forum at eHam.net

$\'\'$

$\'\'$

$\'\'$

eHam.net Forum : TowerTalk : Multiband antenna question

Forum Help

1-5 of 5 messages

Page 1 of 1

Multiband antenna question	Reply
by KC0RDG on June 27, 2006	Mail this to a friend!
I have a question concerning multiband antennas such as the Fan Dipole or any antenna which uses a Gamma T match. It seems to me that in a fan dipole you have 1 feed line to a main post (I'll call it a post, not sure what the technical term is) and from there, you have dipoles cut for each freq. They are all tied together. My question is, when you transmit, isn't the whole antenna radiating? I mean, lets say you have one cut for 20m-6m. When you key up on 20, how does the antenna 'know' to route the RF through the 20m dipole and not radiate RF through the 6m element? I know it's not really 'routing' it as I'm sure the 6m element is getting some RF too. Then it would seem to me that the whole antenna is radiating! Since your outputting RF on 20m into the antenna, the 20m dipole is resonant and thus power is in fact going out the 6m element too but at a high SWR? I'm unclear how the RF 'knows' which element to choose. I hope I explained this well enough. Thanks! kc0rdg

RE: Multiband antenna question	Reply
by K7KBN on June 27, 2006	Mail this to a friend!
RF current, just like DC current, takes the "path of least resistance" -- or "least impedance", in the case of RF. With a multiband dipole, the element that's closest to resonance at the transmitting frequency will have the lowest impedance. The other elements will "feel" the RF a bit, but their impedances will range from a few hundred ohms to a few thousand ohms. Compare this with the resonant element, which will be somewhere in the 50-75 ohm range. There is always some interaction between/among the elements for the various bands, but the resonant one is the one that will be doing 99% of the radiating. The others may radiate a small amount, but at the same frequency.

RE: Multiband antenna question	Reply
by WB6BYU on June 27, 2006	Mail this to a friend!
Imagine having 5 resistors across a battery. 4 of them are 1K and the 5th is 50 ohms. The 50 ohm resistor will carry 20 times the current and therefore 20 times the power of the other resistors. So 95% of your power is dissipated by the one resistor. If you have multiple dipoles on a common feedline it is the same situation, except that the impedances contain reactance in addition to resistance. But one of the elements will have a low impedance and the others will have a high impedance, so most of the power will be radiated by the antenna tuned for that frequency. Now consider the case where you have separate dipoles cut for 40m and for 15m: when operating on 15m, that dipole will have an impedance around 50 ohms and the 40m wire will be around 100 ohms. So 1/3 of your power gets radiated from a different wire. Is that a problem? It will change your radiation pattern somewhat, but not enough to notice. You'll still hear and work stations, but they may be a bit stronger or weaker depending on their direction relative to the antenna. So, no, it isn't a problem.

RE: Multiband antenna question	Reply
by KC0RDG on June 27, 2006	Mail this to a friend!
Makes total sense to me now. Thanks for the explanations!

RE: Multiband antenna question	Reply
by WB6BYU on June 28, 2006	Mail this to a friend!
I should add that a Gamma match, or T match (they are different) is a totally different condition. In that case the matching network may look like a very LOW impedance on the bands where the antenna is not resonant. Putting several antennas of that type in parallel will NOT work well - the SWR will probably be high on all bands.

Page 1 of 1

Next Topic: Rohn Documentation
Previous Topic: Balun or Tuner?

Email Subscription
You are not subscribed to this topic.

Check our help page for help using Forum, or send questions, comments, or suggestions to the Forum Manager.

$\'\'$