If you are involved in any sort of radio transmission, you probably have at least heard of SWR or standing wave ratio. Most transmitters can measure it these days and most ham radio operators have tuners that measure it, also. But what are you measuring? [KI8R] points out that if your coax has loss — and what coax doesn’t? — you are probably getting an artificially low reading by measuring at the transmitter.
The reason is that most common SWR-measuring instruments pick up voltage. If you measure, for example, 10V going out and 1V going back, you’d assume some SWR from that. But suppose your coax loses half the voltage (just to make an obvious example; if your coax loses half the voltage, you need new coax).
Now, you really have 5V getting to your antenna, and it returns 2V. The loss will affect the return voltage just like the forward voltage. Reflecting 2V from 5 is a very different proposition from reflecting 1V out of 10!
On the other hand, as [KI8R] points out, SWR isn’t everything. In the old days, you’d load your transmitter’s finals into just about anything. Now, solid-state rigs expect to drive a low SWR, or they will crank down the power to prevent the reverse voltage from damaging them.
Overall, it is a good talk about a subject that is often taken for granted. Of course, with cheap VNAs, you can easily measure SWR right at the antenna, often with disappointing results. If you have trouble visualizing standing waves, we know someone who can help.
[Quote]if your coax loses half the voltage, you need new coax[/quote]
… or are working on really long lines, which is really not that bad if you know what,you’re working with
3 dBV loss is quite ok, unless the cable is really short.
You calculate your link budget over antenna, cable, pre-amp and transceiver both in terms of signal strength/SNR and noise figure, both on sending and receiving side. Only when that does not sudfice, you decide where to put your money and effirt for improvement.
A single voltage drop says nothing.
At least please use something like dB/m that is attenuation/loss per distance when talking about transmission line quality.
You can make a high power dummy load for transmitter testing by coiling up some coax and terminating it with a low power 50 ohms resistor.
Turns out if you coil up enough coax, you might as well skip the resistor.
So the SWR of the coax+antenna is different to the SWR of the antenna by itself. This shouldn’t be a surprise, and it doesn’t mean the SWR at the transmitter is in any way wrong. It just means that you can’t rely purely on the SWR reading to determine if your setup is good.
I.e., the fact that not much power comes back to where your transmitter is, doesn’t necessarily entail that all the power that left that transmitter was radiated by the antenna.
High SWR is bad for your transmitter. Low SWR doesn’t necessarily mean you’re “getting out”.
The solution in that is to use the readily available information from the Internet.
Services like: https://pskreporter.info/mac.html
I built a magnetic loop antenna, used a VNA to tune the antenna (it’s extremely low bandwidth), and then used pskreporter to verify my signal was making it around the world, particularly since my physical setup is substantially less than optimal. (Apartment / 90 degree field of view, concrete building)
The Reverse Beacon Network is quite useful as well
Exactly. The article says “you are probably getting an artificially low reading by measuring at the transmitter”, but that is false. The reading you get at the transmitter is accurate at the transmitter, which by the way is all that the transmitter actually cares about. How much power is actually reaching the antenna is another story, but it is a fallacy to equate the two.
Soon after I started wirh ham radio in 2009 someone showed me a “magicx antenne with a low SWR over a suspicously big range with a suspiciously simple design, which had a big blob of resin as it’s central construction element. The manufacturer would not gie away a hint for th secret sauce he used.
After someone steped up with an x-ray, showing a single 50 ohm resistor connected to two random “antenna” wires, the magic was gone.
I learned: SWR is not efficiency.
And: An antenna that is not reciprocal (same emitting an receiving behavoiur) should clearly explain what it is actually doing.
There is no voltage coming back to your finals, the problem is that they get way too hot due to an uneven load.