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Does a low SWR mean your antenna is a great performer?


George N2APB
 

FYI … I was recently in a roundtable on 75M and a perennial topic was discussed at some length ... "If my antenna has an SWR of 1:1 is it a great performer?"

 

The answer is simple ("not necessarily") but the reasoning is summarized well in the following post from the ham.stackexchange list.

 

I thought this would be of interest to all, but mostly for those on the HF bands where the science of homebrewing antennas is more often practiced.

 

72, George N2APB

 

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What is the relationship between SWR and receive performance?

 

If an antenna analyzer shows 1:1, does that mean it's an ideal receiver as well?

 

No.

 

Assuming we're talking about a characteristic impedance of 50 ohms, a 50 ohm resistor (otherwise known as a dummy load) will show a SWR of 1:1, although it will almost certainly perform very poorly as either a receive or transmit antenna.

 

The low SWR simply tells you that there are no impedance mismatches along the path from the transmitter (antenna analyzer in the case of your question) to the antenna feedpoint, at the current operating frequency.

 

And what about the converse, will a well performing receive antenna show a 1:1 SWR?

 

Yes and no.

 

Yes, a SWR of 1:1 means that you aren't losing signal to impedance mismatch reflections.

 

No, another issue is how efficient the antenna is at picking up the (desired) signal, preferably (especially in the case of directional antennas) while rejecting undesired signals as well. An antenna that is 5% of a full half-length dipole isn't going to pick up as much RF as the full-length dipole, let alone a full-sized directional antenna pointed in the proper direction, simply due to the much smaller physical (antenna aperture) size.

 

Generally speaking … if an antenna analyzer or (other) transmitter shows that the antenna output presents a SWR of 1:1, then what you have is probably about as good as it gets. That does not necessarily mean that what you have is a good antenna setup as exemplified by the extreme example of a dummy load.


John Haskell
 

What George quoted from ham.stackexchange is correct.

To take it a bit further, SWR can be pretty much ignored.  If your solid state transmitter functions with normal output power versus shutting down, then SWR really does not matter much for most installations.  This is particularly true at HF where cable loss is minimal.  At UHF and with a long transmission line SWR can cause unacceptable losses.  

Here are a couple of examples of SWR versus loss using inexpensive RG-58.

Assume 7MHz and 75 feet of RG-58.  The cable loss is .9dB with a SWR of 1.

But what if the SWR is 2:1?  The extra loss due to this SWR is .2dB, e.g., negligible.


For a mobile set up on 442.1MHz..   Many people go nuts with low loss cables.  Not me. I use RG-58 in the car.  Here is why.  For 442MHz, 10feet of cable, and a 2:1 SWR, the additional loss due to the 2:1 SWR versus a perfect match is .2dB.  I do not believe you will notice that extra loss.


I have been considering putting a transmission line/SWR presentation together for TLARC in hopes of dispelling some of the common myths.  If there is interest let me know.  Maybe that will get me off my butt.

73, K1AT


WK9M
 

Yes I think that a presentation would be a great idea--TLARC is looking for new presentations.  I myself put in memory the 4-11-25% rule.  i.e. 1.5:1 is 4% loss, 2:1 is 11% loss, and 3:1 is 25% loss.  I found a document online that confirms my memory is correct:
http://www.packetradio.com/pdfzips/SWRvsPowerNwatts.pdf

And as you point out, 2:1 is a .5 dB loss.  Even 3:1 is only a 1/5 of an S-unit (1.25dB loss); I'd say that a computer can hear that difference but probably not a human.

Many modern radios will start cutting power back at even moderate VSWR levels.  So as you say, watch the power out.  Both of my Kenwood HF radios are picky on 6m for example.

VSWR can have a bad effect on surge arrestors if you happen to buy a low power one and operate it with high VSWR.  (3:1 is literally 3x the voltage).  But I doubt any of us would do that, at least on purpose.  I went over that in my surge presentation about a year ago.

'73,
Randy

On 9/21/2020 12:48 PM, John Haskell via groups.io wrote:
What George quoted from ham.stackexchange is correct.

To take it a bit further, SWR can be pretty much ignored.  If your solid state transmitter functions with normal output power versus shutting down, then SWR really does not matter much for most installations.  This is particularly true at HF where cable loss is minimal.  At UHF and with a long transmission line SWR can cause unacceptable losses.  

Here are a couple of examples of SWR versus loss using inexpensive RG-58.

Assume 7MHz and 75 feet of RG-58.  The cable loss is .9dB with a SWR of 1.

But what if the SWR is 2:1?  The extra loss due to this SWR is .2dB, e.g., negligible.


For a mobile set up on 442.1MHz..   Many people go nuts with low loss cables.  Not me. I use RG-58 in the car.  Here is why.  For 442MHz, 10feet of cable, and a 2:1 SWR, the additional loss due to the 2:1 SWR versus a perfect match is .2dB.  I do not believe you will notice that extra loss.


I have been considering putting a transmission line/SWR presentation together for TLARC in hopes of dispelling some of the common myths.  If there is interest let me know.  Maybe that will get me off my butt.

73, K1AT


John Haskell
 

Randy,

Thanks for your thoughts.  Good point on the peak voltage being higher with a non-perfect match.

The  4-11-25 "rule" works well if the line is very lossy.

I must say though that the 4-11-25 "rule" doesn't quite cut it for low loss transmission lines.

For example, consider a lossless line with a 2:1 SWR.  What percentage of the transmitter's power is radiated?  Answer:  All the power is radiated [not 90%].

What about with a 10:1 SWR?  The loss is again zero!   All the power is radiated.  Think open wire or ribbon line.  This is an important concept.  Even with a high SWR,  100% of the power is radiated.

What really happens is the reflected power is re-reflected at the transmitter [near 100% of it] and is not lost to dissipation in the transmitter.  The reflected power, when it reaches the transmitter, is reflected back in the forward direction where much will be radiated when it again reaches the antenna.  Eventually all the power is radiated in spite of an SWR greater than 1 except for the portion dissipated in the transmission line.  Just think of the energy ping-ponging back and forth with some being radiated when the energy hits the antenna each time.

When some line loss is present the reflected power does suffer some loss in the transmission line on the way back and forward, and that is what the additional loss beyond the SWR 1:1 case represents.  As long as the line has, say, less than a dB or two of loss the additional loss caused by SWR can pretty much be ignored.

73,
Allan


WK9M
 

Sounds good Allan; the actual loss depends on transmission line quality and frequency then as we chatted on the phone when I was by Ralph's earlier.  i.e the losses would be much greater with a long run at 440MHz and 3:1 as it bounces back and forth.  Maybe we should just superconducting coax and then the answer is always 100%?  Ok so maybe not practical on that one.  :)

I found the actual article on voltage, VSWR, and surge suppressors that I had talked about during the presentation if anyone is interested.  The formula is (2) on page 4 of this document for the techies in the club:
http://www.arrl.org/files/file/QEX_Next_Issue/2016/July-August/Hinkle.pdf

Vpk=sqrt(100*PWR*SWR)

So on a 100W signal into 50 ohms at 1:1, the common Ohm's law formula states 70.7V (P=(V^2)/R); solving for V.

At 3:1, the voltage isn't 70.7*3=210.9V but 173.2V...a little less than 3X as you noted.

With 1.5kW, it's an amazing 671V.  So that's why you always tune high power.

'73
Randy

On 9/22/2020 2:29 PM, John Haskell via groups.io wrote:
Randy,

Thanks for your thoughts.  Good point on the peak voltage being higher with a non-perfect match.

The  4-11-25 "rule" works well if the line is very lossy.

I must say though that the 4-11-25 "rule" doesn't quite cut it for low loss transmission lines.

For example, consider a lossless line with a 2:1 SWR.  What percentage of the transmitter's power is radiated?  Answer:  All the power is radiated [not 90%].

What about with a 10:1 SWR?  The loss is again zero!   All the power is radiated.  Think open wire or ribbon line.  This is an important concept.  Even with a high SWR,  100% of the power is radiated.

What really happens is the reflected power is re-reflected at the transmitter [near 100% of it] and is not lost to dissipation in the transmitter.  The reflected power, when it reaches the transmitter, is reflected back in the forward direction where much will be radiated when it again reaches the antenna.  Eventually all the power is radiated in spite of an SWR greater than 1 except for the portion dissipated in the transmission line.  Just think of the energy ping-ponging back and forth with some being radiated when the energy hits the antenna each time.

When some line loss is present the reflected power does suffer some loss in the transmission line on the way back and forward, and that is what the additional loss beyond the SWR 1:1 case represents.  As long as the line has, say, less than a dB or two of loss the additional loss caused by SWR can pretty much be ignored.

73,
Allan