b***'s guide to FM antenna connection
Posted by: bazz on 04 April 2004
Apropos of nothing really, but installing my new FM antenna put me in mind of all the small things which can prevent the signal which arrives at your tuner from being a mirror, if slightly attenuated, image of that generated by your antenna.
I’m not a professional in this field, but I do have a fair bit of experience and have learnt from my mistakes over the years.
I'll assume that you have a purpose-built roof or loft mounted FM antenna pointed at the transmitter, and that you don’t use 300 ohm ribbon to connect it to the tuner. (Ribbon actually suffers from much less signal attenuation than coax, but is horribly prone to RFI and multipath distortion)
If you are able to install an external Yagi then mad if you don’t. An indoor antenna or external half wave dipole will deliver but a fraction of your tuners potential.
Start with the balun, the little gizmo usually found at the antenna end of the cable which converts the balanced 300 ohm output of the antenna to the unbalanced 75 ohms impedance of coaxial cable and (probably) your tuner input. Most antennas come with a PCB balun already attached but some (such as mine) don’t. If you have to buy one, spend a couple of extra dollars/quid to get the best one in the shop as, believe it or not, an inefficient balun will absorb around 80% of the signal fed into it.
When you attach the balun to the antenna, make sure the flexible wires are kept equidistant between the attachment points on the antenna’s driven element and the balun body to maintain 300ohms impedance.
Coaxial cable: Turf that old RG59 coax in the shed. Use quality double screened RG59, RG6 or, preferably, quad screened RG6 as used in satellite TV installations. The quad screened stuff is a bit thicker and a bit awkward to work with, but worth the effort (heard that before somewhere?)
Coax relies on the screen to keep interference out and signal in. I have seen cheap RG59 with 20% braid coverage, which is hopelessly inadequate. The best coax has alternate layers of copper foil and braid screen with an air spaced dielectric (insulator) and a copper coated steel conductor.
Make sure too that the braid and foil are made of the same material. If they're not, electrolytic corrosion will occur when the cable becomes damp. Have a look at coax which uses differing screen materials after six months use and you’ll see that the conductor is black from corrosion. Alternatively, attach a zinc anode (just kidding!)
Also, when you’re terminating the coax with a balun or plug, make sure that no part of the screen comes into contact with the conductor, not even one stray strand of braid. (This may seem obvious, but I’ve seen some strange things when helping friends with their installations.)
Cable attenuation: Not really an issue at FM frequencies and with relatively short (say less than 50m) cable runs. Still worth keeping in mind though that the difference in attenuation between the best and worst cable is around 6db over 100 metres at 100 Mhz and we’re dealing with signals measured in microvolts.
Cable Routing: Don’t run the coax alongside AC mains cable. Even the best coax is only 85% effective at rejecting RFI/multipath/induced interference. If you have to cross a mains cable, do it at right angles and try to maintain some clearance.
Cable Joins: Avoid them. A join will result in reflections/standing waves, which will certainly compromise the VSWR the engineer who designed your antenna spent sleepless nights achieving. The same applies to wall plates, though I realise they are often a domestic necessity.
RF (booster) amplifiers: Avoid them at all costs. They will introduce noise and distortion to the signal and they kill music stone dead on contact. If you have a weak signal get a more efficient antenna.
Plugs: I personally prefer F connectors, but if your tuner uses a Belling Lee (PAL) type, then use one you can solder.
If all this sounds inconsequential I assure you it's not. The difference between cheap RG59 and quad screened RG6 for example is clearly audible in my system, and I only have 14 metres of cable between antenna and tuner.
I’m not a professional in this field, but I do have a fair bit of experience and have learnt from my mistakes over the years.
I'll assume that you have a purpose-built roof or loft mounted FM antenna pointed at the transmitter, and that you don’t use 300 ohm ribbon to connect it to the tuner. (Ribbon actually suffers from much less signal attenuation than coax, but is horribly prone to RFI and multipath distortion)
If you are able to install an external Yagi then mad if you don’t. An indoor antenna or external half wave dipole will deliver but a fraction of your tuners potential.
Start with the balun, the little gizmo usually found at the antenna end of the cable which converts the balanced 300 ohm output of the antenna to the unbalanced 75 ohms impedance of coaxial cable and (probably) your tuner input. Most antennas come with a PCB balun already attached but some (such as mine) don’t. If you have to buy one, spend a couple of extra dollars/quid to get the best one in the shop as, believe it or not, an inefficient balun will absorb around 80% of the signal fed into it.
When you attach the balun to the antenna, make sure the flexible wires are kept equidistant between the attachment points on the antenna’s driven element and the balun body to maintain 300ohms impedance.
Coaxial cable: Turf that old RG59 coax in the shed. Use quality double screened RG59, RG6 or, preferably, quad screened RG6 as used in satellite TV installations. The quad screened stuff is a bit thicker and a bit awkward to work with, but worth the effort (heard that before somewhere?)
Coax relies on the screen to keep interference out and signal in. I have seen cheap RG59 with 20% braid coverage, which is hopelessly inadequate. The best coax has alternate layers of copper foil and braid screen with an air spaced dielectric (insulator) and a copper coated steel conductor.
Make sure too that the braid and foil are made of the same material. If they're not, electrolytic corrosion will occur when the cable becomes damp. Have a look at coax which uses differing screen materials after six months use and you’ll see that the conductor is black from corrosion. Alternatively, attach a zinc anode (just kidding!)
Also, when you’re terminating the coax with a balun or plug, make sure that no part of the screen comes into contact with the conductor, not even one stray strand of braid. (This may seem obvious, but I’ve seen some strange things when helping friends with their installations.)
Cable attenuation: Not really an issue at FM frequencies and with relatively short (say less than 50m) cable runs. Still worth keeping in mind though that the difference in attenuation between the best and worst cable is around 6db over 100 metres at 100 Mhz and we’re dealing with signals measured in microvolts.
Cable Routing: Don’t run the coax alongside AC mains cable. Even the best coax is only 85% effective at rejecting RFI/multipath/induced interference. If you have to cross a mains cable, do it at right angles and try to maintain some clearance.
Cable Joins: Avoid them. A join will result in reflections/standing waves, which will certainly compromise the VSWR the engineer who designed your antenna spent sleepless nights achieving. The same applies to wall plates, though I realise they are often a domestic necessity.
RF (booster) amplifiers: Avoid them at all costs. They will introduce noise and distortion to the signal and they kill music stone dead on contact. If you have a weak signal get a more efficient antenna.
Plugs: I personally prefer F connectors, but if your tuner uses a Belling Lee (PAL) type, then use one you can solder.
If all this sounds inconsequential I assure you it's not. The difference between cheap RG59 and quad screened RG6 for example is clearly audible in my system, and I only have 14 metres of cable between antenna and tuner.