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Simple Antennas For AM...

 
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Simple Antennas For AM...
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erobertg
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Joined: 29 Oct 2004
Posts: 113

Post Simple Antennas For AM... Reply with quote
First of all, low power broadcast band transmitters are sometimes avoided because they require tall towers. Actually, to get low angle vertically polarized signals, that's is what's required. What can a single tall omidirectional radiator produce? A certain local station in my area has a daytime output 5 kw and reduced power to a mere 8 watts at night. I live about 10 miles as crow flies from this station and I can pick it up clearly at night with an ordinary cheap radio. I can double the range with a good communications receiver and good antenna. Using this as a frame of reference I have something shoot for.

Perhaps the easiest antenna to setup is a single random wire approximately 100-150 ft. long. The problem single end fed random wire is that it has anywhere from 500-1000 ohm or even more impedance and has to be matched to the transmitters 50 ohm output. The low cost MFJ-16010 does just that. It has 12 positions of switched tapped toroid and a variable capacitor. Several different combinations of inductor/capacitor settings gave me SWR readings under 2.0, the lowest I got 1.1 but my transmitter only putting out 6 watts. My transmitter connected to a dummy load was running 8 watts. My best setting with the tuner gave me a SWR of 1.3 but the transmitter was cranking a full 10 watts from a random wire! Not bad for a single banana jacked wire about 140 ft. long wrapped around my deck and to the nearest tree approximately 8-10 ft. off the ground.

A range check revealed a range of about 3 miles ground wave but I'm sure a horizontal wire provides plenty of skip at night. I'm more interested in local coverage and so antenna is going to get a boost in height. I'm thinking of using a small weather balloon to get the wire vertical. The tuner is basically part of the antenna and can be housed in a sealed Tupperware container at the base outside.

Another option is a helically wound tube or even a couple slinkys. Here the little MFJ tuner is actually used in reverse as the impedance is usually 5-10 ohms for these kinds of antennas. Well, the search continues for the perfect antenna.
Fri Nov 19, 2004 2:37 am View user's profile Send private message
erobertg
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Joined: 29 Oct 2004
Posts: 113

Post Grounding Is Important... Reply with quote
It's vitally important to ground your transmitter. I initially used a wire wrapped around a SO-239/BNC adapter on the back of the transmitter and an alligator clip at the other end. I connected the ground clip connected to a baseboard heater fins. It helped with a static problem I was having and so thought it was a good ground.

Well, I found out it wasn't such a good ground. I could broadcast to the west about 3 miles but to the east I was getting out no more than a mile. I later checked the house wiring and found a well grounded system with a ground rod driven deep into the earth. I connected the transmitter via banana plug into electrical outlet's ground and wow, what a difference. I now get out 5 miles in all directions. Even over a mountain. Not bad for a low 140' random wire/impedance tuner combo. I realize that a broadcast station requires a radial grounding system so opted for an Isotron antenna.

I expect to get my Isotron delivered Friday. The nice thing about Isotrons is they don't require an extensive grounding systems. I'll keep my outlet ground to the transmitter as well as use the ground strap to mast supporting the Isotron. I'll keep you all posted.
Mon Dec 06, 2004 9:39 pm View user's profile Send private message
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