I was looking into antennas for FM transmitters. I was thinking, can a CB radio antenna work for this? I know it is designed for 27ish Mhz AM, would it be different for FM? I plan on transmitting on 96Mhz. Is there any risk for the transmitter? I'll get a J-pole antenna if I have to, but I have a bunch of CB antennas laying around.
If you remove the loading coil of most whip antennas made for cars and trim the length of it to resonate on 96MHz then yes. You can't just use a CB antenna as is unless you want to ruin your transmitters final transistor. It has to be a classic style whip antenna and at least 2.7 feet long. To get the exact length take 234/frequency in mhz. You will want to cut the length a little longer and trim down until SWR is lowest. For example 96.1 would be 2.44 feet, or 2 feet 5 inch. Also make sure there is no capacitors built into the base of the antenna, some of the shorter CB antennas have them.
Another good antenna that works is a car FM full length antenna, just cut to desired frequency and mount to an old CB antenna base, or a homemade one.
These type of antennas work best on a metal surface that is grounded. This is why they work on cars because the metal roof. If you don't have a base, then use a good chunk of sheet metal and stick the magnet base on top of it.
Monopole (whip) antennas have almost no gain so don't expect it to work any better than a basic dipole.
Post by phrettbender on Dec 5, 2009 19:49:28 GMT -6
I need an antenna for about th 93.7 FM range, but would like to have the option to change up to 101.1. Can a folded dipole work in the entire FM spectrum without adjustments? A bit of signal loss is not an issue, just keeping my transmitter safe. I'm looking at the Ramsey TM-100 with all the fixins rather than try to make my own with no definite info on baluns, etc. to make it work well.
Bandwidth can be increased by increasing the conductor diameter. You can use bigger conductors (for that BW, we're talking HVAC pipe), or a simpler solution is a "cage dipole", where several wires are tied together and held open to simulate a very large diameter.
Just wanted to add that circularly polarized antennas work the best for the "FM Broadcast" band for a number of reasons. There is a great little program you can use to design them here... www.ve3sqb.com/cirpol.exe The program tells you how to build one using square tube you can get at most home depots and the like. I've ran 2kw into a bay of 4 of them using square aluminum stock from HD and it works very very well. Just my 2 cents... If you are running under 500w one antenna will work just fine.
The second thing is that you need to get yourself an SWR meter, it measures the power going out and being reflected back to the transmitter. That is how you know your antenna is resonating correctly. These can be had on ebay for a good price, just make sure it covers the bands you need. You could even go a little bit further and get an antenna analyzer and test any antenna for resonance at the FM Broadcast band.
Post by phrettbender on Dec 7, 2009 17:19:27 GMT -6
Thanks for the info. I'm going to build a folded dipole using pvc pipe and 10ga single strand insulated copper wire and also one similar to the circular polarized one but using 1/4" copper tubing for a trial run. Both should be cheap and give me let me do a bit of experimenting in distance, which won't bee too much with my little setup but it's got me involved.
I'm planning on the SWR meter before getting anything up and running. I just need to find what works well in the FM band. I don't need anything fancy just yet, but knowing how I do things that will change! lol
Well, you just have to make sure the meter covers the FM band of 88-108 mhz. As long as it does your good. You also want to make sure it has enough resolution for the power your running. If your only running 5 watts it won't show up well on a meter that only has a scale for 0-300w. You'll want to have one that has a scale somewhere in the range of 0-20 or 30w if that's the case.
As far as your antenna adventure is concerned, the circularly polarized antenna should have gain over any dipole so it should have more radiated power then the dipole if it's constructed correctly. Good luck! Copper pipe works real well for antennas, just make sure you solder the connections to avoid corrosion and moisture issues. A good number of the professional circular antennas for 30kw and over are made of 2" to 5" copper pipe....
Protect freedom, or freedom won't be there to protect you....
Post by phrettbender on Dec 12, 2009 18:11:34 GMT -6
Right now I only have 1 watt of power, and I'm not sure the TX is tweaked to even supply that. I hooked up a folded dipole and did not get very good results. With the other antennas do I need the Balun and choke core, of can the coax just be soldered to the copper and tweaked for SWR?
A folded dipole has an impedance of about 300 ohms so you shouldn't connect it directly to a 50 ohm transmitter. Try a cage dipole instead. There's a program to design them as well.. www.smeter.net/software/dipcage2.exe Use a small choke balun to avoid problems caused by common mode currents.
Yes a folded dipole is 300 ohms of impedance so a direct connection to your transmitter would not be a good idea. Even at one watt you still could overheat the output transistor with that much of a mismatch. You could use a matching transformer to match the 300 ohm to 50 but it's a waste of time. Just go with a basic open ended dipole. They have an impedance of around 70 ohms which is close enough to 50 to work fine in most situations. To remove common mode currents at FM broadcast frequencies a simple method is to take around 5 or 6 turns of coax cable and loop it together about 6" diameter and tape it with some electric tape to hold it firm. This must be done right before the end of the coax connects to your antenna outside. Common mode currents can give you a higher SWR and reduce how well the antenna works greatly so this is is almost necessary when using dipoles.
Post by phrettbender on Dec 15, 2009 18:32:28 GMT -6
The Antenna I have has a proper balun, but I'm now builing a J pole with the same setup you describe with the turns of coax to even things out. I can't say I'm impressed with the Ramsey Tru-Match, but as I live and learn how things work I'll get better I just got notified that my SWR meter is delayed in shipping, so things are going slow
Well, a vertical may still need some type of matching network to the feed line to be a perfect load. Dipoles are very simple antennas, they are not the best! A dipole has only gain over an isotropic antenna (a theoretical antenna). They will not radiate any more power then what you put into them. A good vertical can have around 5db of gain over a dipole (dBD). Even more gain when compared to an isotropic radiator (dBi) (a dipole has 2.14 dB of gain over an isotropic antenna). The more gain you have over a dipole the higher your ERP will be. I support the use of dipoles for people who want something simple and easy but many other antenna designs will radiate much more power. A gain of 5 db is a little less then four times more radiated power. The effort in building an antenna that exhibits gain over a dipole is well worth it if you ask me!
Protect freedom, or freedom won't be there to protect you....
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I just got notified that my SWR meter is delayed in shipping, so things are going slow