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First of all, while I'm not new to electronics, I'm new to the radio world. I am helping a friend overseas set up a community radio, and am getting everything set up over here before I ship it.

I have a CZE-7C FM transmitter, and I have built a simple half wave dipole antenna tuned to the frequency he wishes to use. However, I have noticed in my research that half wave dipoles have a theoretical impedance of about 73 ohms, where as the transmitter wants a 50 ohm antenna. I've read that an ill-matched transmitter and antenna can damage the transmitter, and therefore I would like to know what further preparations are necessary before I hook my simple DIY dipole up to my transmitter.

Perhaps I just don't know what terms to search, but I can't find any advice. I was planning on using 50 ohm coaxial with BNC terminations to connect them, if that helps.

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    \$\begingroup\$ If the antenna is tuned to resonant length, for power levels you are likely to be messing with it should be fine. An SWR meter may be the best means of tuning the antenna, though length formulas from a ham reference should get you close. You have secured legal authorization to operate this, correct? \$\endgroup\$ – Chris Stratton Jan 27 '15 at 19:16
  • \$\begingroup\$ This will be operating on 7 watts. I used an online calculator for half wave dipole antennas to get the lengths for the frequency it's going to be operating on. I should be fine to literally just hook the output onto the two lengths of wire in the antenna with 50 ohm coax? (enclosed in white pic with BNC connector) I am leaving the legal certification to my friend, and he says it's going well. He lives overseas, so I don't have a clue how that works over there. :p I'm just making the thing for him. \$\endgroup\$ – Nimaid Jan 27 '15 at 21:33
  • \$\begingroup\$ I have, since asking this question, been studying intensely to get my Technicians License, and have learned much. If the antenna is truly tuned, then the SWR should be very close to 1:1, regardless of the impedance mismatch. (correct?) However, the transmitter output is an unbalanced line, and a dipole is, by operational principal, balanced. So even if I have a low SWR at my desired, fixed frequency, a 1:1 balun would benefit me. The issue now is this: How do I construct baluns of various ratios? \$\endgroup\$ – Nimaid Jan 28 '15 at 9:08
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    \$\begingroup\$ An antenna doesn't have an SWR - it's the impedance matching that gives you a good or bad swr. \$\endgroup\$ – Andy aka Jan 28 '15 at 9:53
  • \$\begingroup\$ The first question should be: is 75 ohms enough of a bad match to cause damage? Nothing is perfect. Engineering is all about finding good enough. \$\endgroup\$ – Phil Frost Feb 20 '17 at 15:18
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The output is very likely to be an unbalanced type suitable for quarter wave antennas with a matching section. It's got half the impedance of the dipole i.e. 38 ohms and a simple 12 ohm resistor in series will match it to the 50 ohm output.

You can match it to a half wave antenna but you need to use a balun to drive that type of antenna properly - this will also allow you to get the impedance correctly matched.

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  • \$\begingroup\$ In practice, a balun isn't actually needed. \$\endgroup\$ – Chris Stratton Jan 27 '15 at 19:14
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    \$\begingroup\$ en.wikipedia.org/wiki/Dipole_antenna \$\endgroup\$ – Andy aka Jan 27 '15 at 19:26
  • \$\begingroup\$ +1. Bending the dipole to make it more like a V can also serve to get the impedance to 50 ohms. \$\endgroup\$ – Phil Frost Feb 20 '17 at 15:12
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If you need to match the antenna to the transmitter, why not build yourself a Butterworth Low Pass Filter, equip it with trimmers instead of capacitors and use that to get a perfect match. A LPF can also work as a very good antenna matching device as I have discovered by building a few filters myself.

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I'm assuming that when you say "Community Radio", you are speaking of a transmitter operating in the standard FM broadcast band of 88.1 through 107.9 MHz?

I have a couple of transmitters that I use for festival use and, although I purchased a couple of antennae, I built the ones that I use the most often. FWIW - all of my transmitters have a standard 50 Ohm output.

The antennae that I purchased are two distinct styles: one is a monopole, the other looks like a pair of folded dipoles crossed in the exact shape of a "X".

The monopole has an omni pattern, the circularly-polarized antenna has a figure 8 pattern. I'm guessing that the monopole is actually configured as a half-wave dipole but I didn't look inside the (sealed) pole to see how it was built.

Both of these have an external balun in a sealed package - I haven't bothered to open either of them up to see how they are constructed. I will, someday.

The antenna that I use the most often is a simple dipole made with 2- fairly-long telescopic antennae mounted on a small project box. The hot antenna points UP, the ground antenna points Down. No matching network of any sort - both of the telescoping poles simply connect to a BNC connector.

I found a table of length adjustments that allow me to set the telescoping lengths to match the frequency that I am transmitting at. Interestingly enough, the downward-pointing telescoping antenna (ground) is a different length than the upward-pointing antenna.

The table seems to be fairly accurate - I normally get reflected power of less than 1 Watt with a forward power of 30 Watts. I simply tweak the antenna lengths to minimise the amount of reflected power if the antenna is too close to some other grounded object that messes up the tuning.

I normally mount the FM Transmitter antenna from the very top of the Front Of House mix position. There is usually a structure above the mix position that houses two or three spotlights (with their operators) and I'll simply hang the antenna from the roof of that upper structure.

I haven't gone looking lately, but I'd think that there is a ton of information just a short Google search away.

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Why not match the 50 ohms to 73 ohms by using an electrical 1/4 wavelength of 60 ohm cable between the 50 ohm cable and the antenna? The 60 ohms comes from sqrt(73 * 50) and an electrical 1/4 wavelength will convert one impedance to the other. The electrical wavelength is determined from the velocity factor of the cable. Divide the 1/4 wavelength by the velocity factor to get the electrical 1/4 wavelength.

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  • \$\begingroup\$ 60 ohm cable is quite a specialty item. Though I can't see why this wouldn't work, so the downvote doesn't make sense to me. \$\endgroup\$ – Phil Frost Feb 20 '17 at 15:16

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