UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Aerials, Feeders, Transmission lines, AMU's and ATUs. Matching.

Mike. Watterson · 20th Jul 2019, 4:02 pm

Yes, you can absolutely match what the radio sees. But the stupid feeder only is a transmission line (a feeder) if it has the Characteristic impedance at BOTH ends. Otherwise it becomes a radiator, an aerial NOT a transmission line.
This is the commonest mistake made by radio amateurs, "I've got good SWR using the 'ATU' between my feeder and the radio, so the aerial is matched." Unless the feeder is a good bit less than 1/4 wave long you have matched the COMBINATION, NOT the actual aerial and the feeder WILL not be a feeder, it will be the aerial. All you are doing is protecting the PA (finals), you are not matching the aerial.

So, unless the feed line is electrically, short, no the good SWR doesn't mean the Aerial is matched. The Aerial is NOT matched!

Antenna matching is NOT just for the Radio or for transmitting. Yes, "matching" the transmission line + aerial rather than the aerial will let you pick up more signal or transmit full power. However if the line more than 1/10th to 1/8th wave long, the longer it is compared to wavelength, IF the junction of aerial to feed is NOT where it's matched, then the feeder is picking up on receive (more interference recieved) and radiating on transmit (potential interference or damage to your premises).

Do read up on WHY a feeder for an aerial is a transmission line and why a coax won't screen and radiates/picks up and a balanced feed line (300, 450, 600 etc) is an aerial, not a feeder, UNLESS BOTH ENDS are matched with the characteristic impedance.

If the feeder is electrically short or the aerial starts at the back of the ATU (Match box), then a good SWR means the aerial is matched. If the feeder is electrically long, then even perfect SWR ONLY means no reflected signal or mismatch for the RADIO. It's not at all and can't ever match the aerial. Remote ATUs to fit at junction of the aerial and the feed are not a deluxe option or a scam. They mean your feeder is not radiating or picking up!

A feeder is ONLY a feeder if matched at the radio AND the Aerial ends. The Radio end is simple. The Aerial end is simple for VHF, UHF or an aerial with a defined impedance. A G5RV, inverted L or a long wire is unlikely to match at more than one frequency, or maybe at all.
Matching ISN'T just to protect transmitter PAs.

20th Jul 2019, 4:02 pm	#6
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Aerial for old valve sets. Yes, you can absolutely match what the radio sees. But the stupid feeder only is a transmission line (a feeder) if it has the Characteristic impedance at BOTH ends. Otherwise it becomes a radiator, an aerial NOT a transmission line. This is the commonest mistake made by radio amateurs, "I've got good SWR using the 'ATU' between my feeder and the radio, so the aerial is matched." Unless the feeder is a good bit less than 1/4 wave long you have matched the COMBINATION, NOT the actual aerial and the feeder WILL not be a feeder, it will be the aerial. All you are doing is protecting the PA (finals), you are not matching the aerial. So, unless the feed line is electrically, short, no the good SWR doesn't mean the Aerial is matched. The Aerial is NOT matched! Antenna matching is NOT just for the Radio or for transmitting. Yes, "matching" the transmission line + aerial rather than the aerial will let you pick up more signal or transmit full power. However if the line more than 1/10th to 1/8th wave long, the longer it is compared to wavelength, IF the junction of aerial to feed is NOT where it's matched, then the feeder is picking up on receive (more interference recieved) and radiating on transmit (potential interference or damage to your premises). Do read up on WHY a feeder for an aerial is a transmission line and why a coax won't screen and radiates/picks up and a balanced feed line (300, 450, 600 etc) is an aerial, not a feeder, UNLESS BOTH ENDS are matched with the characteristic impedance. If the feeder is electrically short or the aerial starts at the back of the ATU (Match box), then a good SWR means the aerial is matched. If the feeder is electrically long, then even perfect SWR ONLY means no reflected signal or mismatch for the RADIO. It's not at all and can't ever match the aerial. Remote ATUs to fit at junction of the aerial and the feed are not a deluxe option or a scam. They mean your feeder is not radiating or picking up! A feeder is ONLY a feeder if matched at the radio AND the Aerial ends. The Radio end is simple. The Aerial end is simple for VHF, UHF or an aerial with a defined impedance. A G5RV, inverted L or a long wire is unlikely to match at more than one frequency, or maybe at all. Matching ISN'T just to protect transmitter PAs. Last edited by Mike. Watterson; 20th Jul 2019 at 4:18 pm.