FM100B with Homemade Dipole - Balun or Not?

[Dangerously]

I made myself a dipole out of a few things I had laying around the house; 30" of coathanger wire on each side into 50-ohm coax. Crude, but it seems to work very well (actually I'm surprised at how well it works).

My question is do I need to balance (or otherwise modify) this antenna somehow to keep from damaging the FM100B? If I need to modify it, what exactly do I need to do? If it need a balun of some sort, how can I construct it?

If I need to throw away the whole antenna and build another it's no big deal (only took me about 15 mins to build this one, so no big loss) - just give me exact intructions on how to build a proper one. Do I need to go as far as getting an SWR meter and tuning the antenna, or is that going too far for this low-power transmitter?

Thanks!

PS. I want to build a ground-plane antenna, but until I can get a spot to mount one, this dipole will have to do, so I want to make sure the transmitter won't be damaged by the dipole.

[Mark W]

Dangerously,
You are asking a lot of questions here, and they all depend on one question you have to ask yourself. What kind of coverage are you attempting on achieving? If you want maximum coverage, by all means look into our PM10DC power meter. It is probably one of the most cost-effective methods for tuning antennas in the micro power range. This will allow you to simply take some wire-cutters to the ends of your dipole and trim them to length while watching the VSWR for best match.

A balun is mostly useful for reducing energy from being reflected down the outside of the cable and wasted because it is radiated from the shielding of the cable. It instead forces that energy to be expended in the antenna elements and improves match somewhat. This is mostly true with lousy coax cable. Expensive low-loss cable with good shielding does not need this as much. Unfortunately you wont be able to tell if the balun is doing you much good unless you have the equipment to check it. Again the PM10DC would help.

Otherwise use the old tried and true method of adding in the balun, checking the range, trim the ends, check the range, and so on. A lot more work, but it will save you the money you would otherwise have to spring for equipment.

-Mark W

[ac5dk]

I built a dipole similar in design to yours out of an old scanner antenna that I trimmed up. I used an MFJ antenna analyzer to tune it, but the micro power meter will effectively do the same and costs less. At these power levels, you're not likely to damage the radio. Tuning is more about getting max signal out the antenna. I DID find that tuning my antenna improved my coverage area.

This kind of dipole antenna actually has a feedpoint impedence of 75 ohms. And since the transmitter probably doesn't care much between 50 or 75 ohms, you might actually do better going with some really good RG-6 coax and it might even be cheaper. (Just my humble opinion.) Anyway, I went that route and found via the analyzer that the match on the antenna was better with the 75 ohm coax, (1.05:1) but it might differ with the radio, so the power meter would be the best way to check.

I was shopping one day and saw a TEKK brand home indoor receiving antenna. It had in it two transformers... the type used to convert 75 ohm coax to twinlead hookups on your TV. Only one of them (it was red) was a 75 to 75 ohm unit. Since, I've never seen such a balun before (at least I think it's a balun - why else would you have a transformer to convert to the same impedence) I bought the antenna just to get it (~$10). And I gave the antenna away to a neighbor (and now listener) that had 300 ohm terminals on his stereo and didn't need the 75 ohm termination. Apparently many of your newer home stereos have 75 ohm hookups instead of 300 ohm. Who knew?

I'm sorry I don't remember the model number, but I know that it's available on the web and several versions of their antennas include this balun. It would be great if Ramsey could buy some to resell. I know they are meant for reception, but it would surely rate out for 25mW. And BTW, it has a red and black wire... Hook the red wire up to the element that goes on top; it's the center conductor.

You will get better gain out of a dipole (2.14 DBi) than a 1/4 wave ground plane (0 to 1 DBi depending on height above ground). Tell me what frequency you are operating on and I can tell you how long to make the coat hangers. The formula for a halfwave dipole is 468 divided by the frequency in MHz. But that makes feet, so you have to convert to inches.

Example: 95.5 Mhz => 468/95.5 = 4.900 feet or 58.8 inches... for the WHOLE thing. Each side would then be 29.8 or just say 30 inches. And that's from feedpoint (the place where the center of the coax is no longer covered by the outer shield) to the ends. AND this is for average wire or coat hangers; if you were to use aluminum tubing (like I did) it'd be a little bit shorter because the "wire" has more surface area.


How about a chart:

Freq ---- total -- per side
88 MHz -- 64" ---- 32"
90 Mhz -- 62.5" -- 31.25"
92 MHz -- 61" ---- 30.5"
94 Mhz -- 59.75" - 29.65"
96 MHz -- 58.5" -- 29.25"
98 Mhz -- 57.25" - 28.75"
100 Mhz - 56" ---- 28"
102 Mhz - 55" ---- 27.5"
104 Mhz - 54" ---- 27"
106 Mhz - 53" ---- 26.5"
108 Mhz - 52" ---- 26"

I rounded off a little because frankly it's not THAT critical. You can extrapolate for frequencies inbetween. This assumes a wire or coathanger size diameter AND gives a 75 ohm feedpoint, so RG-59, RG-6 or RG-11 coax is wunderbar, depending on length of run.

That ought to get you going Dangerously... if you put it up outside, don't forget to weatherproof the coax connection.

Happy Broadcasting!

Kevin AC5DK
PowerFM (95.5 FM)

[ February 23, 2003: Message edited by: PowerFM ]

[stumped again]

I hooked up a TV type dipole to my FM10 and used your figures, PowerFM to adjust the height of the sides. The result was no better than the stock little whip the kit came with. But when I extended each side with stiff wire to about 45 inches, I transmitted over twice as far! Are the figures in your chart for the maximum legal range only? Stumped again, Toledo, Ohio <BLOCKQUOTE>quote:</font><HR>Originally posted by PowerFM:
I built a dipole similar in design to yours out of an old scanner antenna that I trimmed up. I used an MFJ antenna analyzer to tune it, but the micro power meter will effectively do the same and costs less. At these power levels, you're not likely to damage the radio. Tuning is more about getting max signal out the antenna. I DID find that tuning my antenna improved my coverage area.

This kind of dipole antenna actually has a feedpoint impedence of 75 ohms. And since the transmitter probably doesn't care much between 50 or 75 ohms, you might actually do better going with some really good RG-6 coax and it might even be cheaper. (Just my humble opinion.) Anyway, I went that route and found via the analyzer that the match on the antenna was better with the 75 ohm coax, (1.05:1) but it might differ with the radio, so the power meter would be the best way to check.

I was shopping one day and saw a TEKK brand home indoor receiving antenna. It had in it two transformers... the type used to convert 75 ohm coax to twinlead hookups on your TV. Only one of them (it was red) was a 75 to 75 ohm unit. Since, I've never seen such a balun before (at least I think it's a balun - why else would you have a transformer to convert to the same impedence) I bought the antenna just to get it (~$10). And I gave the antenna away to a neighbor (and now listener) that had 300 ohm terminals on his stereo and didn't need the 75 ohm termination. Apparently many of your newer home stereos have 75 ohm hookups instead of 300 ohm. Who knew?

I'm sorry I don't remember the model number, but I know that it's available on the web and several versions of their antennas include this balun. It would be great if Ramsey could buy some to resell. I know they are meant for reception, but it would surely rate out for 25mW. And BTW, it has a red and black wire... Hook the red wire up to the element that goes on top; it's the center conductor.

You will get better gain out of a dipole (2.14 DBi) than a 1/4 wave ground plane (0 to 1 DBi depending on height above ground). Tell me what frequency you are operating on and I can tell you how long to make the coat hangers. The formula for a halfwave dipole is 468 divided by the frequency in MHz. But that makes feet, so you have to convert to inches.

Example: 95.5 Mhz =&gt; 468/95.5 = 4.900 feet or 58.8 inches... for the WHOLE thing. Each side would then be 29.8 or just say 30 inches. And that's from feedpoint (the place where the center of the coax is no longer covered by the outer shield) to the ends. AND this is for average wire or coat hangers; if you were to use aluminum tubing (like I did) it'd be a little bit shorter because the "wire" has more surface area.


How about a chart:

Freq ---- total -- per side
88 MHz -- 64" ---- 32"
90 Mhz -- 62.5" -- 31.25"
92 MHz -- 61" ---- 30.5"
94 Mhz -- 59.75" - 29.65"
96 MHz -- 58.5" -- 29.25"
98 Mhz -- 57.25" - 28.75"
100 Mhz - 56" ---- 28"
102 Mhz - 55" ---- 27.5"
104 Mhz - 54" ---- 27"
106 Mhz - 53" ---- 26.5"
108 Mhz - 52" ---- 26"

I rounded off a little because frankly it's not THAT critical. You can extrapolate for frequencies inbetween. This assumes a wire or coathanger size diameter AND gives a 75 ohm feedpoint, so RG-59, RG-6 or RG-11 coax is wunderbar, depending on length of run.

That ought to get you going Dangerously... if you put it up outside, don't forget to weatherproof the coax connection.

Happy Broadcasting!

Kevin AC5DK
PowerFM (95.5 FM)

[ February 23, 2003: Message edited by: PowerFM ]<HR></BLOCKQUOTE>

[ March 21, 2003: Message edited by: stumped again ]

[ March 21, 2003: Message edited by: stumped again ]

[ac5dk]

Well, my figures are for a standard halfwave dipole to achieve a 1:1 match... not really sure what you mean by a "TV type" dipole. THAT could have something to do with it... as in what impedence is at the feedpoint, etc. (Is it fed with coax or flat twinlead?) If it's a 300 ohm impedence, then no you aren't going to do much better than the whip with those figures. AND you don't mention what frequency you are using, it could be that you are creating a double 5/8 wave antenna or something. Something interesting to note would be whether you are getting twice the range in ALL directions with the 45" wires.

Antennas are about experimenting and what works. Tell us more about your antenna and setup and maybe we can add some more suggestions to help! :D

Happy Broadcasting!!!

Kevin
PowerFM (95.5 MHz)

[stumped again]

To Power FM: Thanks for your reply! By TV type dipole, I meant I am using a standard "V" shaped VHF dipole that comes with TV's such as 19" when you buy them. I have my FM10 transmitter tuned to 106.9. The antenna has 300 ohm twinlead, hooked up to a 300 to 75 ohm Radio Shack adaptor, which is connected to the RCA type antenna jack in the back of the transmitter through a 75 ohm to RCA jack Radio Shack adaptor. So, I am wondering how can the 2' 2-1/4" dipole lengths per the type of calculations in your chart be the most efficient length to give maximum range, since 45" lengths are working better. The range seemed about the same in any direction with either length, though I didn't try to measure off the distances. Stumped Again, Toledo, Ohio

[ac5dk]

The fact that it's fed with 300 ohm twinlead means that those figures won't fly. The type of dipole I meant is fed with 75 ohm (or 50 ohm) coaxial cable, and one "leg" is straight up and the other down, unlike your "vee". You may be coming up with some multiple of wavelengths, but hey, if it's working for you... :D It would be interesting to see what the SWR is at the transmitter, but you'd have to have a meter to measure it. MORE stuff to buy!!! ;)

HB!!!

Kevin
PowerFM (95.5 MHz)

[ March 22, 2003: Message edited by: PowerFM ]