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Forum Index : Electronics : RF: Does SWR matter with low output power?
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9307 |
I have a small FM transmitter here, that I am going to use at my local village to re-transmit a talkback station they can only get on AM, which is a horrible signal and full of static and buzzing etc. I will use iHeartRadio app on a cheap cell-phone to stream the station, and re-transmit on the FM band. The transmitter will go inside the roof cavity, but I don't have a pole to put a proper antenna on, and one won't be getting installed as it is a concrete tile roof, so there is nothing I can mount a pole to. So, I am going to simply use the whip that came with the transmitter, but when I put it on my SWR meter just for a bit of a look-see, the SWR was terrible. Basically 100% mismatch.   The transmitter outputs 500mW FWD, and with the supplied antenna, I have 500mW of REF power too! Yuk!  However, I need a SMALL antenna that can be installed inside the roof cavity, so I am just wondering how serious SWR is with respect to low output power. I normally only deal with FM transmitters have a few watts of output power, and a correctly matched external antenna. With higher output transmitters, if the SWR is really bad, the reflected power can easily cook the transmitter output final, so this is where I am coming from. How likely do you think I am, to cook the final in the transmitter, with a 500mW output that is a complete mismatch with the supplied antenna? Even if I DON'T cook the final, I would expect with that bad a reflected power, the actual range of the transmitted signal would be far from what you should be able to get with 500mW, yes? Effectively zero range I would have thought, if your FWD and REF figures are the same - one completely cancels out the other!  Perhaps I can shoehorn a J-pole into the roof space. They are quite easy to make and tune, and at the 107MHz end of the dial, they are shorter then at the 88MHz end, so... Or maybe the 'ole copper-rod and SO239 socket shaped into a quarter-wave ground-plane antenna..... Hell, even a wet bit of string would probably be better then the supplied whip!  Smoke makes things work. When the smoke gets out, it stops! |
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TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 6099 |
quarter-wave ground-plane or vertical dipole will be as good as anything. If using the vertical dipole, run the coax away from the dipole at right angles (ie horizontal) for a couple of metres. A high SWR will be very low efficiency so 2 (or 3) to 1 is preferred. High SWR will increase the feedline losses. It's not worth chasing anything better than 2 to 1 for your purposes. Jim VK7JH MMedit  MMBasic Help |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
The final will probably be fine dissipating an extra 500mW. But you will probably be able to measure the transmission range with a wooden school ruler. Build yourself a proper ground plane antenna, and a tweakable pi coupler antenna tuner box at the transmitter end, and match that 500mW bad boy into whatever horror antenna and feeder you can rig up. VK3ALY. Edited 2020-08-26 17:31 by Warpspeed Cheers, Tony. |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
I would be more concerned about the actual RF voltage on the output device under high VSWR and that needs to be checked at all phase angles. Also, is the transmitter stable under all phase angles of reflected voltage? Unless, the transmitter has been tested at high VSWR for these conditions I would ensure that a reasonably low VSWR is placed on it. Dave |
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wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 1156 |
Edit - whoops just noticed Warp & Tassy Jim already suggested ground plane fix. Edited 2020-08-27 07:39 by wiseguy If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9307 |
Excellent info, thanks chums.  I will see if I can whip up a GP or a dipole. Both are easy to make and small-ish. J-pole would be better, but it is bigger so I don't think I could fit it inside the roof-space. I like J-poles or Slim-Jim's, as they represent a DC short, and so they seem to be able to cope much better with any kind of static discharge. I have had a few transmitters killed by corona discharges into open-ended GP's, but never had that problem again with J'pole/Slim-Jim. Oh, and if you want to play a bit of a game, give a J-pole or Slim-Jim to an electrician, and with a 20W or so RF output, put a bulb across the antenna so it lights with the RF energy, and ask him or her to explain to you how the bulb can light, with a dead-short across the coax!  ....and old RF engineers trick that was taught to me when I was learning - and stumped me too as well at the time, till you stop thinking in DC terms.  (IE: A J-pole etc IS a DC short, but it is not a short-circuit at RF frequencies.) Smoke makes things work. When the smoke gets out, it stops! |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6099 |
J pole if you have room but it's a bit long for most roof spaces. You can bend it so the matching stub portion is horizontal and only the half wave section is vertical. There is negligible radiation from the matching section. I used to make Slim-Jims out of 300 ohm TV ribbon. They fit in your pocket and hang from any convenient support. If you want to play with light bulbs, try running one along an open wire feeder looking for the nulls in the standing-wave. Jim VK7JH MMedit  MMBasic Help |
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