Hey,
I thought a while whether I should write this or not. Primary because I have no intentions of annoying someone. But, well, I think I should say the following...
I've always had dipoles give high SWR when using a stock fifty feet length of coax as the feed. I've found a length (...)
Thank you for writing this. There is no use in voicing statements like "
it always works" or "
this is the perfect antenna".
A phrase like "
this always works" will only lead to one thing: the freshman feeling stupid because he can't just get a simple works-always solution working. He'll feel frustrated. Just remember when we started? How many "
why does that not work, damn?!"- moments did we live through? Especially with such complex things like antennas. And, well, ask 5 HAMS, gets 7 answers (which is fine. Everyone can only reflect from personal experiences, which differ greatly).
Let me explain why:
- An antenna exists (as experienced user know) of a an inductive (L, +j), capatitive (C, -j)and two ohmic parts (Wire loss Rl and radiation resistance Rr). Those parameters are prone to be affected by a lot of factors. Do we know about those factors at the place where the feshman wants to build his antenna? Hardly.
- Of course we can say such like "start with a wire that's a bit too long and cut it inch by inch until you hit the best SWR". But likely the poor guy will never obtain a good SWR. Why?
- The reactive L/C parts need to cancel each other out. That's the case when the wire is cut to the perfect length. But a perfect length does not grantee a perfect SWR because the feedpoint impedance may still be way off our target (lets say that's Z=50R). So the poor lad can start with 200 feet of wire and end up cutting it down to 1 inch and will still wonder "why I didn't get a good SWR? People say 'I just need to cut it down until I get a perfect SWR'".
- Maybe the freshman obtains a perfect SWR with testing the antenna 3m above the ground (so he can work on it with a ladder). As soon as he puts it up to 10m the SWR goes postal again. He doesn't know why and will be frustrated. Why? Because (we assume L/C cancel each other out) the feedpoint impedance swings (on a graph) up and down with rising height of the antenna. At some point it's 50R for sure. But likely it well be something between 20 and 80R.
- Maybe he uses 7m of Coax when experimenting and ends up with a good SWR. As soon as he uses 12m of Coax the SWR runs out of scale. Why? Because, yes, like Pigmeat mentioned, Coax can work as a impedance transformation line (look up "stub lines" or "stub transformator" on HAM literature).
But likely, the poor freshman faces some impedance like 36R+j155 and he has noooooo idea how to get that to R=50Z because no one gave him a kickstart with antennas but just "
this always works".
So, since I'm a HAM I'll add my own opinion to that now (I still have to stick to stereotypes).
So, what's the solution? No "
just do this, it'll always work" for sure.
Of course, one of the best ways to get the freshman started is to provide him with some links to basic Antenna theory and advice him to puracse an antenna analyser (yeah, owies, several hundred bucks). But that way he can experiment and verify the theory from the book by results from the antenna analyser. And he can learn what stuff affects the antenna and will get it on track.
But, yeah. You're right to laugh at me for saying that. That's not realistic.
So my advice is: yes, use a (not complex, self build) Antenna Tuner. The setup is as follows:
[TX]-- Z=50R COAX-LINE -->[C-L-C Tuner] -> [BalUn] -> Dipole
Or, if it's prefered to have the tuner inhouse, he could use something like a G5RV-Antenna.
You may ask "
why the tuner?". Well, like I wrote above, there's no way to know about the parameters of the antenna and a tuner is the simplest way to compensate any L/C-part and also transform the impedance to 50R.
Kind greetings (and really, I didn't mean to step on anyones tail if I did by accident),
Zazzle[/list]
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