Antenna forum?

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Antenna forum?

Post by Dion on Fri May 06, 2011 1:26 am

Maybe I should try to figure this out from books, but I suppose I'm looking for a shortcut. I have several radios from the 20's, 30's, and 40's, and all either require or would benefit from an external antenna. Maybe there is enough interest to justify and support an antenna forum? I'll give you examples of the types of questions I have: 1) Many sets have a hot chasis, and I've measured some pretty high voltages between antenna terminals and ground. So if I hook up an external antenna to these this terminal aren't I creating an electrocution hazard? 2) If I'm targetting a 1/4 wavelength antenna, do I have to put up a bunch of different antennae to cover the various portions of broadcast and shortwave bands? 3) Do the drop leads (from antenna to radio) count as part of the antenna length? If there is not enough interest to justify a forum, is there someone who might be willing to mentor me or answer a lot of silly questions on the subject? -Dion Wink
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Re: Antenna forum?

Post by Bill Cahill on Fri May 06, 2011 9:40 am

Unfortunately, there isn't enough interest to merit putting in an antenna forum at this time. You can post it under either radio repair, or elctrical, mechanical.
Hmm. Do we have that one? If not, I may add that.
To answer your questions, for the most part, you don't need different types of antennas, just different directions.
Obviously, the longer the wire, the better the reception.
I highly reccommend against using an external antenna, and, especially a ground, on ac-dc sets. This is an ectreme shock hazzard!
Plus, if you accidentally reverse the plug on such sets, you risk fire as well.
I once did a real no, no, once, even though I had switches on the antenna circuits.
I once had several 20's battery radios playing, and, connected to one antenna as an attempt to save wire.
With all A batteries connected, I inadvertantly casused a short on one of the A batteries, and, it destroyed the battery. It spat acid all over the place!
Ends up all the sets had different arangements of what was grounded. One radio specifically had a positive ground instead of a negative ground. That caused a short on that A supply.
I now use seperate antenna systems.
AC DC sets, that is radios without a power transformer, were never intended to be grounded, and, at best, a wire in the house.
Just too manny dangers...
Hope this helps.
Bill Cahill

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Re: Antenna forum?

Post by Resistance is Futile on Fri May 06, 2011 12:45 pm

One thing that would help, would be to put a capacitor in series with the antenna to block DC, the value depending on the frequency.
If it a general coverage receiver, in most instances a random length of wire is OK. You can however string more than one wire of the proper band length on the same connection at the same time and the closest length will resonate producing a stronger signal.

You can use either the entire length of the external antenna to receive, or you can use coax cut to the band you want with the antenna connected to the coax acting as a counter poise antenna. This is more practical in the higher frequencies however.

You could mount a "T" type antenna one element would be used for ground connection and the other as the receiving antenna.
As you know, you can actually put a loading coil in line with a receiver antenna and electrically lengthen the antenna even though its shorter physically .

1/4 wave antennas are a great beginning but 5/8 wave antennas are considered better due to gain.
You could also use baluns to balance the antennas, or design an antenna switching network or scheme. And have an antenna tuner in-line with the receiver antenna

Antennas are a science all in themselves.
Hope this helps a little.
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Re: Antenna forum?

Post by Resistance is Futile on Fri May 06, 2011 1:08 pm

Oh yes!
Another thing you could do if you want to safely use one antenna for several sets is to design a coil for each radio and loop it around the radio coil without making a direct electrical connection. That would also isolate them all electrically from each other.
That may load your antenna though unless it were designed with a switching network in mind.

This could be moved to our communication receivers and boat anchors section, or the tutorials and tips forum. But I agree with Bill there isn't enough interest in serious antenna discussions at present to warrant a separate forum topic. IMHO
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Re: Antenna forum?

Post by Dion on Tue May 17, 2011 6:14 pm

Thank you for the replies. Any suggestions for a practical primer on the subject of antenna design and connection? -Dion
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Re: Antenna forum?

Post by Brig on Wed Feb 08, 2012 6:39 pm

Dion wrote:Thank you for the replies. Any suggestions for a practical primer on the subject of antenna design and connection? -Dion

Almost any "treatise" dealing with radio repair and restoration will contain information about antennas. Here are three suggestions:

W. Marcus and A. Levy. Elements of Radio Servicing,1947, chapter 17

Markus, John. Television and Radio Repairing, 1961, chapter 23 (note author's name is spelled with a k; not the same as Marcus above with a c)

A. Ghirardi and J. R. Johnson. Receiver Circuitry and Operation , 1951, chapter 12


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Re: Antenna forum?

Post by Bill Cahill on Wed Feb 08, 2012 7:44 pm

Thanks for the helpful info.
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Re: Antenna forum?

Post by Dion on Fri Feb 10, 2012 11:52 pm

Brig: Thank you for the book suggestions. I have been off-line for a few months, but finally made it back to my radios. I have been trying to digest Joseph J. Carr's "Practical Antenna Handbook", but a lot of it is over my head, mathematically. I just want to know what's going to work, with some practical examples. I'll finish the Carr book, but will check out your suggestions.
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