FM Micropower update

terrydec · Posted: Nov Tue 03, 2009 7:42 pm

Okay, I bought the Ramsey 25B. I noticed that they introduced a 30B and a 35B. The 30 looks like the 25 with a bunch of goober stuff tacked on, and the 35B is 1 watt, but expensive. What I need to know is this- while I wait for the kit, does anyone have a good plan for a ground plane. I have 6 good adjustable elements. Can you have too many radials? I was thinking of brasing, (?) the ground elements to a copper ring, with the vertical element held off with an insulator. Will this work? Should I just use 3 or all 5 elements? How do I terminate something like this, and how do I match the impedance? Do I have a clue Question

HAHA

boogieman · Member Joined: 27 May 2009 Posts: 55 Location: Flint, MI

For a ground plane, the vertical radiator is connected directly to the center conductor of the coax. There's no matching needed as it's an unbalanced antenna.

However... I've only built 50 ohm antennas, but the coax you are using is around 75 ohms. Ramsey doesn't say squat about the output impedance in the manual, but the "F" connector would tend to indicate 75 ohms.

For a 50 ohm ground plane, I always used a 1/4 wave radiator above a counterpoise of 4 radials spaced 90 degrees apart, angled about 135 degrees from the radiator.

Feedpoint impedance increases with the angle of the radials. Radials 90 degrees to the radiator generally has a low impedance around 35-ish ohms. Dropping them to 135 degrees brings the impedance to about 50 ohms. Beyond 135 degrees opens up to debate. The impedance will increase but it will (in theory) act as a coaxial dipole rather than a ground plane.

In reality... I've had a 1/4 wave radiator above a counterpoise angled around 150 degrees and it worked fine. Looked like a miniature Starduster. I got out close to a mile on 50mW at about 15 foot above ground. It was mounted on a second story apartment balcony, and had a wide null in the directions the building blocked. I would have loved to get it on a rooftop to see how far that 50mW would go. In the unobscured directions, I got at least a mile.

The height of the antenna make a difference as well. As the height increases, a mushroom develops upward in the center. YMMV.

Here is a link for calculating your lengths: http://www.csgnetwork.com/antennagpcalc.html

My advice as far as impedance matching... angle the radials at 150 degrees from the radiator and cross your fingers. I've never seen a 75 ohm SWR meter, so as far as checking that... I am at a loss.

The 75 ohm output (if it actually is) is going to be tricky to load into a ground plane though. You might consider the antenna Mike Yancey linked to in the other thread. According to Ramsey's page on that antenna, it's a 75 ohm antenna. Maybe that would work better for you.

terrydec · Posted: Nov Wed 04, 2009 3:42 am

Yeah, playing around with the antenna's going to be interesting all right. I like the suggestion about the angle. My experience has always been that you can usually go from low to higher impedance with less loss than from high to low. I bought a field strength/SWR meter for about $10 on eBay, but at this strength that's pretty much useless. I'm going to try the digital cable. Yeah, I know-RG 8 is best, and I have some with PL256 connectors; but the transmitter will be at the back door, so it should reach, and the '25 IS 75 ohm out. I'm going to be right at the peak of the roof. One area I'm trying to reach is across the street, and my porch is at the back. Well, like everyone says, height IS everything in this situation.

terrydec · Posted: Nov Wed 04, 2009 3:52 am

More- I just looked at the manual for that antenna. They use steel wire "Ladder" twin lead for that thing. I haven't seen that stuff in years! I can remember trying to make it fit under a window. If it twisted in the wind it would raise the window. It's really hard to work with, and It'll bite ya too. However, that said, it sure is efficient. Anybody know where I can find some? That's probably why they charge so much for it!

terrydec · Posted: Nov Wed 04, 2009 4:41 am

FOUND IT!!!!!
I think
Page 81 of the AES catalogue. It's almost impossible to read, but it says Ladder Wire, in 100' rolls for $19.99
(I think)
Now THAT stuff is the real deal.
In the manual -Ramsey says that this antenna is their stock answer to getting the most distance. It even gives enough direction that anyone here could build it. The hard part is building the balun, but anyone as smart as us should have no problem with that.
It's almost worth the $70 they want for it!
I'm going to contact AES tomorrow and make sure it's the same stuff!!
Yiippee!!
I've got my antenna, (I hope)
Nite all-
t.

marko · Member Joined: 05 May 2009 Posts: 462 Location: canada

What sort of ladder line though, it comes in different impedances...
The stuff used on TV's was 300ohm, i think it's more common around.

What's wrong with coax..?

terrydec · Posted: Nov Wed 04, 2009 4:56 pm

I found it from a guy in Ontario. I'm waiting for a quote for 10' w/ S&H. Yeah, it's 300- and I made sure it's the stuff like chicken wire, not like what came with your Marantz Laughing

terrydec · Posted: Nov Wed 04, 2009 5:10 pm

More- you should check out this antenna-

http://www.ramseyelectronics.c ... /TM100.pdf

It is balanced, comes out of a PVC enclousure and goes into 75, which matches the 25 output and is highly efficient. The main advantage is that it won't require a lot of tweaking and configuratin'. It's pretty much midrange, but the secret is the wire. I used to use this stuff way out in the country.The main problem came from interference if someone drove by in a pickup- about 5 times a day- we're talking hills so steep that the animals have 2 short legs, and can only go around in one direction. Haha