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golfguruPosted - 21 December 2012 20:22  Show Profile  Email Poster  Edit Message  
Just wondering.
It appears that a MW antenna <120feet long will be capacitive across the range.

Could one erect a <240feet (inline) longwire, with centre feed, to still obtain a capacitive antenna having double the capture area (re: nodes and all that stuff)?

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Edited by - golfguru on 12/21/2012 8:24:13 PM

_J_Posted - 21 December 2012 20:48  Show Profile  Email Poster  Edit Message
Hey Golf,
"Capture area" is somewhat of a myth, the antenna's conductor does not have to subtend the entire wave to take the power from it. The truth is that a 1 inch antenna has the same "capture area" as a quarter wave Marconi. In practice, what makes it impractical is not its "small capture area" but that its impedance is almost unmatchable, you waste all the power trying to match to it. If you could match to it with lossless parts, it would receive just as well.

With that said, back to your question. The vertical lead-in for the inverted L or the T does the heavy lifting for MW waves. Thats because MW waves down here near the earth are mostly vertical polarized. The horizontal part mostly just adds capacitance, which in the end makes it easier to match to.

So I am not sure where you measure the 120, I suppose some is vertical and some is horizontal? If you keep the vertical the same and add an equal horizontal part to make a T, it will have more capacitance and require less L to tune it. (For unequal sides on top of the T you can get two resonances.) If more C means you loose less power matching it, then it will 'work' better. I am not sure that is necessarily true or significant though.

John Davidson

Edited by - _J_ on 12/21/2012 8:56:53 PM

golfguruPosted - 21 December 2012 21:0  Show Profile  Email Poster  Edit Message  
>>> (For unequal sides on top of the T you can get two resonances.)

Mmmm... might be better to avoid the problem.

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homebrewPosted - 22 December 2012 8:37  Show Profile  Email Poster  Edit Message  
Antenna experiments are always worthwhile and easy enough to set up as temporary fair weather tests. Each will have a different pick up pattern and impedance. It is a good way to ID a weak fading station that your main antenna just cannot pull in good enough. And occasionally you will pick up a new DX loud and clear off a strong lobe.
So far other antenna designs run 10 to 30 percent less signal than the inverted L longwire on the local boomer.
gsparksPosted - 22 December 2012 10:56  Show Profile  Email Poster  Edit Message
I learned something like this with my first transmitter at the beach shack I had a dipole for 40 meters that loaded well. I decided to try 80 meter, so I connected the center and shield and connected it to my tuner to the feed as a single wire It would not load, arced quite a bit in the tuner. I called my elmer and he told me I didn't have an 80 meter antenna, but I had a center fed antenna that resonated around 7 Mhz, again reminding me to plot the voltage / current lobes starting at the end. moving the feed to the end it would load, didn't work well, but loaded.

I guess the lesson learned was the impedance of the feed point and match was what mattered. and could be changed with feed point location, why the carolina windom works

By the way in High school in electronics class the instructor wrote the word impedance on the board and asked if anyone could explain, an outgoing guy said, he always wondered what It was pronouncing it as impotence. it took the instructor a long while to regain his composure.

_J_Posted - 23 December 2012 1:6  Show Profile  Email Poster  Edit Message
Re: Mmmm... might be better to avoid the problem.

Naah! If you get the sides within a couple yards of equal you will be fine. Antenna tuning wont be that sharp.

John Davidson

golfguruPosted - 23 December 2012 1:41  Show Profile  Email Poster  Edit Message  
Thanks every one - but keep it coming if there is more.

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homebrewPosted - 24 December 2012 16:18  Show Profile  Email Poster  Edit Message  
Moving the feed line from the end to 1/4 and 1/2 down the line changes the pick up pattern on my 150 ft longwire.
Usually worthwhile when I'm looking for max DX numbers. With a sloped antenna the lead in height and length definitely act as a major part of the antenna system.
golfguruPosted - 29 December 2012 18:16  Show Profile  Email Poster  Edit Message  
>>>>>>>>> Hey Golf,
"Capture area" is somewhat of a myth, the antenna's conductor does not have to subtend the entire wave to take the power from it. The truth is that a 1 inch antenna has the same "capture area" as a quarter wave Marconi. In practice, what makes it impractical is not its "small capture area" but that its impedance is almost unmatchable, you waste all the power trying to match to it. If you could match to it with lossless parts, it would receive just as well.

With that said, back to your question. The vertical lead-in for the inverted L or the T does the heavy lifting for MW waves. Thats because MW waves down here near the earth are mostly vertical polarized. The horizontal part mostly just adds capacitance, which in the end makes it easier to match to.
>>>>>>>>>>>

This seems to bring into question why anyone would want a huge longwire? (HB says different lobes, so maybe for that reason?).

One would be better off going higher than longer?

Or is there a payoff with greater "power" from the long wire? (if there is no better signal capture).

A high, <1/4 wave antenna with plenty of capacitive top hat (2 or 3 parallel runs) might be as good as anything on the MW band.?

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Edited by - golfguru on 12/29/2012 6:21:54 PM

homebrewPosted - 30 December 2012 12:30  Show Profile  Email Poster  Edit Message  
My 300 ft longwave antenna here did not do as well as my 150 ft longwave on most stations. The exception was the stations off the tie in end.
Wire losses and impedance mismatches probably have a lot to do with it.
When I lived in the country I put up a 400 foot longwire 10 foot above a run of metal fences, a combination of barbed wire and chain link. It was also highly directional off the ends but gave good reception off the sides also.
My experiments in height here proved to me that the rule of thumb "90 percent of the height of surrounding trees and buildings is optimal" is true. At least for horizontal antennas.
_J_Posted - 31 December 2012 18:0  Show Profile  Email Poster  Edit Message
The reason for having a long wire (either long in the vertical direction or long in the horiz direction) is because you are able to load to it, to get power from it.

Re: "One would be better off going higher than longer?" Yes, higher (up to a quarter wave) is a bunch better, but a bunch harder.

John Davidson

golfguruPosted - 31 December 2012 18:42  Show Profile  Email Poster  Edit Message  
golfguru said:
>>>>> Or is there a payoff with greater "power" from the long wire? >>>>

_J_ said:
>>>> you are able to load to it, to get power from it. >>>>>

golfguru replies:
"You beauty", I finally had an insightful cerebration ...

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Edited by - golfguru on 12/31/2012 6:44:34 PM

_J_Posted - 31 December 2012 21:47  Show Profile  Email Poster  Edit Message
Yea!!
Cool.

So, IN THEORY you can get the same power from a short wire, but in practice, in real life, you really can't because the antenna matching circuit is not realizable without superconductors and superinsulators.

So, yea. Congratulations dude.

John Davidson

_J_Posted - 31 December 2012 21:52  Show Profile  Email Poster  Edit Message
By the way, I didn't mean to imply you can't go a little over a quarter wave long, but you might loose a little because you'd have to use a capacitor to load to it, and that might cost you a smigion of power.

John Davidson

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