Is there any practical way to match the output of my PA which is 420 Ω to my antenna which is 50 Ω at 434MHz?

I was thinking about adding a pi-network and using a VSWR Meter with the output of the pi-network connected to the VSWR Meter and the other end of the VSWR Meter to the antenna, then by trial and error changing components until I get close to 1.00 VSWR.

I have a spectrum analyzer but I do not have a Vector Network Analyzer to create the Smith Charts.

Does anyone know of a better/simpler way?

  • $\begingroup$ Insert a quarter wave section of feedline with 145 Ohm characteristic impedance. $\endgroup$ – captcha Jul 12 '16 at 5:24
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    $\begingroup$ Your PA output impedance is 420 ohms -- weird. What is it? $\endgroup$ – K7PEH Jul 12 '16 at 5:26
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    $\begingroup$ Wait... You don't need to use any trial and error or measurement equipment at all! The beauty of the Smith chart is that you can use it with a compass and ruler to calculate a matching network in less than a minute. There are multiple ways to use it depending on the type of network you want. $\endgroup$ – Andrew W. Jul 13 '16 at 18:56
  • $\begingroup$ I just bought a VNA... It's probably the right way to go. I will still let you know how I do with the VSWR meter. $\endgroup$ – scrafy Jul 14 '16 at 4:08
  • $\begingroup$ You can do the match with a couple of transmission line stubs also, ideally fabricated into the PCB. Might be easier than trying to deal with all the parasitics in doing it with SMT components. $\endgroup$ – Phil Frost - W8II Jul 15 '16 at 22:26

If your output need 420 Ohm (balanced) And your antenna is 50 Ohm (unbalanced)

You could run a 450 Ohm balanced feeder, which are commercially avaiable for cheap.

And put a 9:1 balun at the antenna end, to feed your antenna.

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Comment turned into answer.

You can easily buy a 6:1 impedance transformer to do the job. That gets you close -- indeed, for me and my equipment (ham radio that is) it is close enough. I bought a 6:1 transformer for about \$60 or \$70 from DXEngineering maybe 10 years ago. Have since sold it at a ham-fest.

I am assuming that your question is about amateur "ham" radio in making the following comments.

Along the output from your transmitting equipment (PA or whatever) you have multiple opportunities for a mis-match. This involves the transmission line connected to your equipment, anything in that path such as antenna couplers, and the antenna itself.

You want to do two things: (1) get the best match practical for the antenna and frequency of operation; and, (2) have the lowest practical loss path.

Usually any SWR under 2:1 meets the needs of (1). Above 2:1 and especially over 3:1 you definitely need some type of matching solution such as an antenna coupler, transmission line transformer, and so forth. Most modern amateur radio transceivers will fold back power levels when the SWR seen at its output is above 2:1.

For item (2) above, you want low-loss transmission line for most if not all of the path from transmitting equipment to antenna. For my wire antenna covering bands 80, 40, and 30 I use ladder line except for the last 15 feet. Ladder line is coupled to 50-ohm coax (LMR400) using a 4:1 transforming balun. The loss on the remaining coax is insignificant even when I operate at QRP levels (which I often do). I also use an antenna coupler/tuner (Elecraft KAT500).

For my mini-beam antenna (bands 20 - 10 meters) I use LMR400 coax for the entire path because the matching impedance with the antenna itself ranges between 1.1:1 and 1.8:1 (which I consider very good).

Note LMR400 coax is not the cheapest coaxial transmission line but it is low loss.

Very curious about your PA 420 ohm output impedance. In the last 55 years of ham radio activity, I have never seen any amateur radio equipment, even home brew, that did not have a 50-ohm output impedance. Again, I am assuming amateur radio as the OP question did not make it clear if it is something else.

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    $\begingroup$ Also noting that high output impedance actually is a bad thing, at least in theory. $\endgroup$ – Marcus Müller Jul 12 '16 at 18:41
  • $\begingroup$ High impedance itself is not bad. The impedance mismatch is what is bad. You can model and antenna (with NEC2/NEC4) and have a high-impedance feed (end wire for example). If you do not have a problem with matching then the antenna itself is quite efficient, operates just like you want. $\endgroup$ – K7PEH Jul 12 '16 at 19:15
  • $\begingroup$ wasn't it so that research from 1929 (by Bell labs) found that the best impedance for maximum power transfer was 30 ohms and best impedance for least signal loss was 60 ohms? With 50 ohms they struck the balance to get the best of both worlds. $\endgroup$ – captcha Jul 12 '16 at 22:15
  • $\begingroup$ @K7PEH well, considering a free space/vacuum impedance of 377 Ohm, I'd actually be interested in the design of a transmission line with a 420 Ohm impedance, but maybe I'm just tired $\endgroup$ – Marcus Müller Jul 12 '16 at 22:19
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    $\begingroup$ @captcha -- you are correct about the study you mention but you left out the most important part. The study was to arrive at the least loss for coaxial cable as a long haul transmission line used primarily for telephone trunk lines. Not all transmission lines used by ham radio operators are coaxial. Indeed, before WWII, the most dominant transmission line was open air ladder line with nominal impedance of about 600 ohms. This is a very low loss transmission line and it was used to antennas of all types regardless of their resonant impedance. $\endgroup$ – K7PEH Jul 12 '16 at 23:30

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