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Old 8th May 2008, 05:42 PM   #11
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Quote:
Originally posted by unclejed613
a good passive way to get better reception inside the building would be to have a high gain antenna outside the building and connect it to a high gain fm antenna inside the building, a pair of dipoles would also work, but not as well, but dipoles are easily built.

You know, I thought about that but it seemed too simple to work, I thought it would require some elaborate power setup. Okay, point me to a tutorial on dipoles or high gain antenna construction. I'd Google it, but I'm sure somebody on this forum knows right where to find a good design.
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Old 8th May 2008, 05:49 PM   #12
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here's an fm dipole made with 300 ohm twinlead.
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Old 8th May 2008, 06:00 PM   #13
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Quote:
Originally posted by SharpyWarpy
Will I need to build two of these for stereo? Also the article says I will need a pre-emphasis circuit, I'm assuming for my purposes the line circuit. Do I need to build one of each for each audio signal, left and right?
Would I be able to run a coax from the outside antenna to a radio set close to where he works and use one of those small transmitters from there? Would this require an antenna booster?
Hi sharpy,
You would need to build two of these for stereo.
A pre-emphasis circuit helps improve the signal to noise ratio by amplifying lower-level high frequency sounds more than the low frequency sounds before transmission. You would then need suitable de-emphasis in your sons reciever to have an opposig effect and thus a flat frequency response. I think in America you use a time-constant of 75uS (micro seconds) with a break frequency of 2100Hz.

Quote:
Originally posted by SharpyWarpy



You know, I thought about that but it seemed too simple to work, I thought it would require some elaborate power setup. Okay, point me to a tutorial on dipoles or high gain antenna construction. I'd Google it, but I'm sure somebody on this forum knows right where to find a good design.

If you want a really good book covering everything you need to know about this subject then I would highly recommend Basic Radio by Ian Poole published by Newnes, ISBN 0-7506-2632.

Thanks
Gareth
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Old 8th May 2008, 09:29 PM   #14
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a good high gain antenna would be a 5/8 wave vertical. these have about 6db gain over a dipole. google "J-POLE" antennas. these are the simplest, and can also be constructed with 300 ohm twinlead. most of the ones you see on the web are for 2 meters(146Mhz center freq) and would need to be recalculated for FM broadcast (about 100Mhz center freq). it's just shy of 1.5 times the length. try multiplying the lengths given for 2M (146Mhz) by the ratio of the two frequencies (146/100) and it's 1.46. so your dimensions for the antenna would be 1.46 times longer.

j-poles usually need to be adjusted for SWR, but since you're using them for receive-only and a passive radiator, just make sure you make both exactly identical, and you should get good results. if you have a way of testing the SWR of both antennas at 100Mhz, that would be even better. if you end up a MHZ or two, it's ok, as long as both antennas are identical.
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Old 8th May 2008, 09:36 PM   #15
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I was going to suggest the two antenna (passive repeater) idea.

What about putting one of those little TV/FM signal booster amplifiers between the antennas?
I'm assuming the shielding of the metal building would help prevent feedback (oscillation) from the indoor antenna to the outdoor antenna.
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Old 9th May 2008, 11:23 AM   #16
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it would oscillate. those antenna amplifiers have a lot of gain, and to pad one down to the point where it doesn't oscillate would defeat the purpose of having it. a passive system would probably be best. any active system would have to be direct connect to the receiver, or a frequency translator (shift everything up or down 1Mhz) to keep it from oscillating. even if you could get the antennas positioned properly so that an antenna amplifier wouldn't oscillate, all it would take is somebody walking around within a couple of wavelengths of either antenna for it to begin oscillating.
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Old 9th February 2011, 04:50 AM   #17
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ahan! thats the problem everyone is facing while living or working through steel buildings. I hope that everyone's problem should be solved.
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