What exactly is SWR?

[CSL126]

I know that an SWR reading of 1:1 is the best, and the higher it gets the more signal is lost. What exactly do the numbers 1 and 1 represent though? Are standing waves just RF energy that doesn't get to the antenna? Also, I know SWR greatly effects transmitters, but does it effect recievers at all? Thanks in advance...

 

[flyingwolf]

SWR stands for Standing Wave Ratio.

Think of it this way.
1000 people are in a theatre. You yell fire. only one door is available.

Some of those people are going to get out. But many of them are going to be stuck inside there. Those people are gonan get real hot from that fire (just like your finals).

Now if there was 1 door for each person. (1:1 = a 1 to 1 ratio). Then everyone would get out and no one would get hot.

Overly simplified but I think it works.

As far as affecting receivers, Im not 100% positive of that, though I have seen some that say yes and some that say no.

 

[Pro-95]

Are standing waves just RF energy that doesn't get to the antenna? Also, I know SWR greatly effects transmitters, but does it effect recievers at all?

Yes and No.

Yes, Energy dispersed in an unplanned or undesireable fasion resulting from improper matching or incorrect electrical length of transmission lines = SWR. The problem is the energy must go somewhere and for a transmitter the place to go is the antenna. High SWR will result in radiation of energy to take place in the transmission line or in the radio itself.

You have two sine waves (voltage and current, 180* out of phase from each other) being emitted from a transmitter. If the end of the transmission line is where both the waves are crossing at 0, the middle between the high and low, meaning neither is at it's maximum nor at it's minimum then the SWR is the lowest at that point along the transmission line. Hopefully this is the antenna, the desired place for the energy to be dispersed and the radiation to take place.

And No, a reciever isn't generating any (significantly measureable) electrical energy or standing waves. To a receiver only, everthing and anything connected to the radio is an antenna. From a hunk of wire to your kid brother, if it works, that's all that matters.

 

[rcvmo]

I look at it as a water pump emitting 200 Lbs/in into a pipe that's only rated for 100 Lbs/in. There's gonna be a back pressure that causes the pump to overheat.

As for the reciever, if your wanting to tune the 800 MHz band, and 800 MHz band only, as we're all headed anyways, then i'd get the proper 800 MHz antenna for your listening enjoyment with the most quality. You just can't lose. I understand this may not be the case for multi-band scanning, as for those that live in apartments or so.

Rcvmo

 

[RevGary]

If anyone still has any of the 20+ year old "Vanco" CB antenna matchers, or similar (two dial) and is using 52 ohm cable to your receiver, the values of the two ultra small capacitors across the internal variable capacitors can be decreased to a 10pf@1KV value and this will broaden the resonance of the matcher to cover most of the HF spectrum. A good way to LISTEN to how good it works is to go to the WWV transmitter at 5.000 MHZ. Just tweak the two controls on the matcher and you will be surprised at how well it peaks most any HF antenna for receiving purposes. We were able to gain 7 "S" units on the meter on our National NC 121 testing a 50 foot long wire (with 300 ohm to 52 ohm balun) and a 6 meter beam (receive only). It has to be readjusted for optimum reception as the receiving frequencies are changed but really makes a difference in increasing the signal to noise ratio and pulling out weak signals that might otherwise be lost. We have not tested this with any transmitters and are not suggesting that it will be of help in that mode. Our experience with this slightly modified SWR matcher is in receive mode only.

 

[kb2vxa]

Oh good grief Charlie Brown,

In the singles column you might find a Single White Rat. (With 2 you get egg roll.)

Seriously, the key word is RATIO, the relationship of forward power (what the transmitter sends up the pipe) to reflected (what the antenna sends back down and is wasted heating the final amplifier). It's rather like the blivit, 10lbs of poop in a 5lb bag.

Sorry Lindsay, I couldn't resist my analogy after reading the others, especially the pooped pumper. Somebody must be a fireman. (;->)

"Also, I know SWR greatly effects transmitters, but does it effect recievers at all?"
No, a receiver is not a transmitter, no power or ratio thereof involved. If there were such a thing as a non resonant receiver the antenna might complain but so far I have only heard transmitters complaning about non resonant antennas.

Oh I'm SUCH a prat! Or maybe a blivit? (;->)

 

[N_Jay]

Oh good grief Charlie Brown,
. . . .
"Also, I know SWR greatly effects transmitters, but does it effect recievers at all?"
No, a receiver is not a transmitter, no power or ratio thereof involved. If there were such a thing as a non resonant receiver the antenna might complain but so far I have only heard transmitters complaning about non resonant antennas.
(;->)

So there is no power traveling down the antenna line to a receiver.

Gosh darn, I thought we measure receive signal strength in dBm (a measurement of power)??? :roll:

So the power coming down to the receiver meets an impedance mismatch but goes right on by (because it is receive power and not transmit power and the mismatch can tell the difference)??? :roll:

So does SWR (or more correctly, an impedance mismatch) make a difference? YES!

Does it make a noticeable difference? Not usually!!! (and this is the important answer)(and why you can use RG-6 on a 50 ohm receive system with little issue). :wink:

 

[Al42]

And No, a reciever isn't generating any (significantly measureable) electrical energy or standing waves. To a receiver only, everthing and anything connected to the radio is an antenna. From a hunk of wire to your kid brother, if it works, that's all that matters.

For a transmitter, an antenna isn't generating any (significantly measureable) electrical energy or standing waves, so ...

In receiving, the antenna is the source and the receiver is the load. SWR is symmetrical - the formula doesn't care which way the signal is flowing. The SWR for equivalent impedances - and the loss - is the same, whether the signal is going from the transmitter to the antenna or from the antenna to the receiver. The only difference is that the antenna doesn't burn out or reduce the signal if it's feeding the wrong impedance.

But, to a transmitter, the "if it works, that's all that matters" rule also applies. A ladies' hairpin makes a good antenna at 14 MHz - if it's fed with the proper antenna tuner and you're not looking for too much distance.

 

[Pro-95]

Does impedance impede signal, umm yeah, as resistance. But I've never seen an SWR meter that doesn't use power to make a measurement. I could be wrong, but I've never seen one.

But maybe you can answer this for me. Why does high end $3000+ transceivers with multiple antenna connections with at least one being passed through an internal tuner also have a receiving only antenna connection that is a audio styled RCA plug that isn't tuned?

 

[N_Jay]

Does impedance impede signal, umm yeah, as resistance. But I've never seen an SWR meter that doesn't use power to make a measurement. I could be wrong, but I've never seen one.

But maybe you can answer this for me. Why does high end $3000+ transceivers with multiple antenna connections with at least one being passed through an internal tuner also have a receiving only antenna connection that is a audio styled RCA plug that isn't tuned?

The "Characteristic Impedance" (Short hand "Impedance") of a transmission line DOES NOT IMPEDE the signal.

That fact that an SWR meeter DESIGNED for use with transmitters of a certain power level (watts) would not be sensitive enough to measure milliwatts should not surprise anyone (well maybe some here). :roll: :lol:

What $3000+ transceiver are you speaking of?

The effect of SWR is the same on the signal, but has different consequences in high power (transmitter) system vs. low power (receiver) systems.

1 - Loss of signal to the far end - Same TX or RX

2 - Reflection of signal back to the near end - Re-radiated in RX, turned into heat in a TX (Bad)

3 - Voltage peaks along the cable - Insignificant in RX, Can cause arcing and destruction of the cable in TX (Bad)

4 - Current peaks within the cable - insignificant in RX, Can cause melting of the cable in TX (Bad)

5 - Radiation from the shield - A little more signal loss (or gain) in RX, can cause interference to other systems in TX (Bad)

 

[jhooten]

But maybe you can answer this for me. Why does high end $3000+ transceivers with multiple antenna connections with at least one being passed through an internal tuner also have a receiving only antenna connection that is a audio styled RCA plug that isn't tuned?

To make it easier to hook up a VHF/UHF/MICROWAVE Transverter.

 

[N_Jay]

To make it easier to hook up a VHF/UHF/MICROWAVE Transverter.

Could it also be that it is an HF Amature tranciever, and a RCA plug (Which is an old RF connector) is not too bad for low power use below 30 MHz? :roll: :roll: :wink:

 

[wa8vzq]

Sorry to disappoint everyone who said "No" but the mismatched antenna/transmission line impedance that creates an SWR for a transmitter has a similar effect on a receiver. In the case of a receiver, the electromagnetic signal is a source of power and it induces a voltage and current flow in the antenna that then moves toward the receiver. Everything that applies in the transmission process applies in the reception process except it is at a much lower power level. (Signal strength is measured in dBm which is a measure of power.) So there is power flow on a receive line and yes, standing waves are developed on the line if the impedances are not matched throughout the system. In this case, the impedance of the antenna needs to match the line impedance and that needs to match the receiver input impedance to prevent SWR on the line. The term reciprocity is used to describe the similarity between how an antenna system works on transmit and receive. Since most receivers have more gain available than is needed to detect a signal so the effect of the SWR on a receiver is not as apparent but it is present.

Regards

Dan
Apple Valley, MN

 

[Pro-95]

One transceiver the Icom 756ProIII, manual is here.

See page 11, item #2. Page 15 shows the RCA not only for a transverter but also for a wire antenna, such as a Marconi.

And yes, this is receive only.

And I am not saying that a properly tuned system is not important or doesn't perform better, it's just not as critical as the results from no tuning wouldn't smoke a reciever nor will it irradiate the neighbors. And if it's "close enough" I doubt anyone could hear the difference if tuned to the gnats behind. And any mismatch would only reduce the signal (by exponetial degrees the further the mismatch) meaning if you got "close enough", any further probably wouldn't matter.

But since we are talking about receive only antennas let me ask this, wouldn't aperture and narrow beam directivity be a deciedly more important factor than adjusting for SWR on a receive only antenna? Not saying that all 3 isn't the best. But meaning that if you could get both aperture and narrow beam in one antenna (with "close enough" tuning), that this would perform, for receiveing, better than one that had only one of those traits and proper tuning? So either really high stacked yagi's (which still really only has one trait - directivity) or much lower to the ground level multi-wavelength rhombics (huge aperture and really tight beams) and Beverages (big aperture and small beams) would be far better for receiving. Even a really long multi-wavelength wire (massive aperture and very tight beam) laid out on the ground pointing in the direction of the desired source, would be a better antenna. Yes? I say this because most DX contest stations all seem to have yagi stacks or quads all on 200' towers for transmitting but also have multi-wavelength closed horizontal loops (1 wavelength above ground), rhombics or Beverages (both relativly close to the ground) for receiving so they can hear better. You can't work what you can't hear, right? And now coming back to topic. Closed horizontal loops are generally not tuned from what I have read except by measurement on a tape ("close enough" = wavelength * velocity factor). And admittedly I have not researched rhombics or Beverages with small tuning stubs ("close enough") other than light reading so I don't know on those. So it seems that for receiving, SWR isn't as much as an issue and of 3 elements (aperture, directivity,SWR) the SWR is probably the least important. Now I will concede that if you are using a dummy load antenna, then the best tuning you can do is by far more important as you have already sacraficed too much already.

Also when modeling receiving antennas such as rhomboids and Beverages isn't negative gain much more desireable than positive gain? If db is a measurement of power then for receiving, negative or less than 0 power is better. Course now someone has to explain negative power to me.

Feel free to correct me as I am still learning and can accept my misconceptions.

 

[N_Jay]

Now you are getting it.

SWR is there, but is not overly important on RX systems.

The fact that it is not important does not make it "not there".

As for negative power, its all in the reference. dB is a RATIO, not an absolute measurement.

Yes you want a LARGE negative number in the receivers sensitivity as you are specifying how far BELOW the reference it still receives, but you want POSITIVE Gain in the antenna system (or at least as little loss as possible/practical).

 

[N_Jay]

Is the horse dead yet? :roll:

Lets kick it again! :lol: :twisted: :lol: :twisted:

 

[Pro-95]

Lets kick it again! :lol: :twisted: :lol: :twisted:

Whack!

Thanks N-Jay.

 

[jhooten]

Could it also be that it is an HF Amature tranciever, and a RCA plug (Which is an old RF connector) is not too bad for low power use below 30 MHz? :roll: :roll: :wink:

I just gave on possibility. The KWM-2 series with the Noise Reduction option had one the was just for the antenna for the noise receiving antenna.

Another use on some units was to allow an outboard preamp between the antenna switch and the first rf amp.

And then there is the connection of a second antenna for space diversity (you need a lot of spae at HF), or to support the second reciever in a transciever capable of full duplex operation (yes there are full duplex HF radios).

There are more but I'll leave it at that incase others wish to contribute.

 

[Al42]

5 - Radiation from the shield - A little more signal loss (or gain) in RX, can cause interference to other systems in TX (Bad)

Can cause signal pickup on the shield in Rx (bad, since the shield isn't tuned).

 

[kb2vxa]

Hi again,

"So there is no power traveling down the antenna line to a receiver."
Correct.

"Gosh darn, I thought we measure receive signal strength in dBm (a measurement of power)?"
No, received signal strength is measured in dBuV, DbW is the measurement of power. I have no idea what you're trying to say, decibels milli without a point of reference is meaningless.

I hope that clears things up a bit N-Jay.

I suggest all you confused people take a course in basic electronics or stick to what the scanner shop has to offer off the shelf rather than trying to impress each other with misinformation. If you can't accept my simple answers to simple questions, sorry, I can't help you.

N-Jay, thanks for trying but...
"Is the horse dead yet?
Lets kick it again!"
Then you wrote;
"There are more but I'll leave it at that incase others wish to contribute."
At first I took you for a masochist but then I noticed you finally figured it was time to quit while you're a head. (;->)

Oh Al, you and N-Jay remind me of the Meissner VFO, you guys just can't stay on frequency. The three of us should go out for a couple of beers and leave these guys trying to figure out what a noise bridge is and what it's used for. Oh Gary, you can come too IF you can answer this one question. What was the standard S meter reference point and which receiver employed the only calibrated meter? For you we'll bring the wine, Christian Brothers is my favorite.

Now don't the rest of you raid the liquor store looking, it's altar wine made in the Finger Lakes district of NY and it's got such a nice kick. Yeah, that's why you only get one sip, no 10-24s in church thank you. (;->)