VSWR....Does it even matter?

[G7RUX]

Does VSWR even matter on a Rx only setup such as a SDS100 and an antenna about 40' high on the back of my house? I recently purchased a nanoVNA and tested my VSWR setting my stimulus to the P25 Phase 2 frequencies 769-775MHZ being that is the ONLY thing I listen to. My VSWR was around 1:1.213

I'm just curious if it even matters being that I do not transmitt. can someone shed some light on this subject please?

Cutting a long story short yes it does, especially if you are interested in listening to weak signals. Efficient power transfer requires low loss (tends to make VSWR look better, unhelpfully) and a good match (low VSWR) to get as much of the received signal transferred into the receiver.

 

[paulears]

Now that is the answer he needs. A damp piece of string is an antenna - just not a very good one!

 

[G7RUX]

Now that is the answer he needs. A damp piece of string is an antenna - just not a very good one!

Get some salt on that wet string and it will likely work better than you might have thought, at least for moderate power levelks up to around 10 MHz.

 

[paulears]

A friend had a disused railway line at the back of his house - a pair of mole grips and top band was pretty good!

 

[dlwtrunked]

So, this is one of those troublesome discussion points.
Antenna systems are reciprocal devices, so parameters affect transmit and receive equally....

That is perhaps not entirely exactly true--depending on ones interpretation of that. The receive noise (both local QRM on the receive frequency and receiver front-end generated) are a factor in receive only. As long as the signal is above the noise level, a good receiver can attempt to add gain to the signal. However, none of those factors are relevant to transmitting. ) So an antenna system that gives a good signal-to-noise ratio may be excellent for receiving but that may not work well at all for transmitting--so saying "equally" can be problematical. (Also, depending on ones definition of "antenna system" things can get complicated--I tend to want to keep "receive system" and "transmit" system in discussing a transciever as two separate things. One cannot even discuss an active receive system treating it as both.)

 

[RU55]

Get some salt on that wet string and it will likely work better than you might have thought, at least for moderate power levelks up to around 10 MHz.

That reminds me of years ago. Living on the coast, within 50 feet of the ocean, with UHF television. The reception would become terrible due to the ocean spray corroding the antenna. Lots of salt deposited from the ocean spray. When our favorite program was about to start we would spray the antenna down with the hose to get the TV reception back. Later, we rigged up a hose going up on the roof and a sprinkler so that we just had to turn on the water to clean the antenna.

 

[Ronnierozier2]

Thanks for all the great answers

 

[G7RUX]

Just in case anyone wants to read up on reciprocity in antennas, here's a good book for you:

Electromagnetic Reciprocity in Antenna Theory

<p><b>Provides a self-contained account on applications of electromagnetic reciprocity theorems to multiport antenna systems</b></p> <p>The reciprocity theorem is among the most intriguing concepts in wave field theory and has become an integral part of almost all standard textbooks on...

www.wiley.com

Tl;DR: antennas are reiprocal devices.
If you add amplifiers, etc then they are not the antenna. Active antennas are an antenna with an added amplifier. Considerations of SNR, etc are *external* to the antenna...

 

[MUTNAV]

That is perhaps not entirely exactly true--depending on ones interpretation of that. The receive noise (both local QRM on the receive frequency and receiver front-end generated) are a factor in receive only. As long as the signal is above the noise level, a good receiver can attempt to add gain to the signal. However, none of those factors are relevant to transmitting. ) So an antenna system that gives a good signal-to-noise ratio may be excellent for receiving but that may not work well at all for transmitting--so saying "equally" can be problematical. (Also, depending on ones definition of "antenna system" things can get complicated--I tend to want to keep "receive system" and "transmit" system in discussing a transciever as two separate things. One cannot even discuss an active receive system treating it as both.)

Right, practical understanding wins
A snipet from
Chapter one from "Receiving Antennas are Different"

Thanks
Joel

 

[prcguy]

Maybe its better to state that antennas "capable of transmitting" are reciprocal on receive. You can't expect that to be true with an AM ferrite stick antenna or an active E-field VLF antenna, etc, because you can't transmit on those. That's like comparing apples to raisins.

Right, practical understanding wins
A snipet from
Chapter one from "Receiving Antennas are Different"
View attachment 129798
Thanks
Joel

 

[TexTAC]

In my experience, VSWR can make a pretty big difference on reception. I have a 40m off-center fed dipole antenna, a Palstar AT500 tuner, and a Yaesu 991A transciever. When I move between bands (80, 40, 20, 17, 15, 12m, etc), there is a big audible difference and improvement in reception when I tune the antenna for each specific band. I can also “see” the improvement on the waterfall scope as background noise and weaker signals appear as the antenna gets tuned. For instance, when I am tuned for 40m and switch to 80m, the 80m band is absolutely deaf until I tune the antenna for 80m. Once tuned, I can hear and see the signals within the band.

As everyone else said, it is absolutely crucial to tune the antenna before transmitting, but in my experience it also improves reception. Curiously, the built in tuner on my transceiver does not affect reception only because it is not situated on the reception side of the circuit. The external tuner however, between the radio and antenna, does.

 

[prcguy]

An OCFD antenna should be resonant on most harmonic bands, so part of 80, 40, 20 and 10m should have a low SWR without a tuner. In this case I think its the tuner "detuning" a band you switch to and tuning just makes it normal again. I'll bet if you bypass the tuner and switch between 40, 20 and 10m you will not hear much difference in noise floor.

In my experience, VSWR can make a pretty big difference on reception. I have a 40m off-center fed dipole antenna, a Palstar AT500 tuner, and a Yaesu 991A transciever. When I move between bands (80, 40, 20, 17, 15, 12m, etc), there is a big audible difference and improvement in reception when I tune the antenna for each specific band. I can also “see” the improvement on the waterfall scope as background noise and weaker signals appear as the antenna gets tuned. For instance, when I am tuned for 40m and switch to 80m, the 80m band is absolutely deaf until I tune the antenna for 80m. Once tuned, I can hear and see the signals within the band.

As everyone else said, it is absolutely crucial to tune the antenna before transmitting, but in my experience it also improves reception. Curiously, the built in tuner on my transceiver does not affect reception only because it is not situated on the reception side of the circuit. The external tuner however, between the radio and antenna, does.

 

[TexTAC]

An OCFD antenna should be resonant on most harmonic bands, so part of 80, 40, 20 and 10m should have a low SWR without a tuner. In this case I think its the tuner "detuning" a band you switch to and tuning just makes it normal again. I'll bet if you bypass the tuner and switch between 40, 20 and 10m you will not hear much difference in noise floor.

My antenna is resonant on 40, 20, and parts of 10 but not on 80 for some reason. I still use the external tuner to fine tune the SWR as low as possible on those bands and can get reasonably low SWR on all the other bands too (80, 17, 15, and 12). 160m is off limits even with the external tuner. It isn’t tunable for me so no joy there.

 

[dlwtrunked]

Maybe its better to state that antennas "capable of transmitting" are reciprocal on receive. You can't expect that to be true with an AM ferrite stick antenna or an active E-field VLF antenna, etc, because you can't transmit on those. That's like comparing apples to raisins.

I still have an issue that that itself does not consider local interference. This is why a long wire or dipole, despite how well tuned, may not work well for receiving (but be excellent for transmitting) when a loop that is small antenna (or loop) can be positioned to counter the noise. In effect, the local environment must be considered part of the antenna system (yes, I realize you may already being doing that consideration, but the general audience may not).

 

[prcguy]

Local noise is a separate issue and can vary with location. Antennas intended for transmitting have properties that will be identical on receive and if there is more gain or a particular pattern, that will be the same on receive. Interference or signals on the receive frequency have no bearing on the antenna pattern, gain, impedance, VSWR or other things that relate to its transmit performance.

If you are comparing receive only antennas then yes, an antenna with lower noise pickup could work better than one that is prone to noise pickup.

QUOTE="dlwtrunked, post: 3748754, member: 3288"]
I still have an issue that that itself does not consider local interference. This is why a long wire or dipole, despite how well tuned, may not work well for receiving (but be excellent for transmitting) when a loop that is small antenna (or loop) can be positioned to counter the noise. In effect, the local environment must be considered part of the antenna system (yes, I realize you may already being doing that consideration, but the general audience may not).
[/QUOTE]

 

[G7RUX]

Maybe its better to state that antennas "capable of transmitting" are reciprocal on receive. You can't expect that to be true with an AM ferrite stick antenna or an active E-field VLF antenna, etc, because you can't transmit on those. That's like comparing apples to raisins.

You most certainly can use ferrite antennas for transmitting, although power levels and saturation are an issue if you are not careful. An "active" antenna is generally an antenna element (very small in the case you indicate) with an amplifier iattached thereafter; the antenna itself is reciprocal.