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Hi all,
I have a NanoVNA, great little gadget, I'm interested in in aerials, at present for receiving only.

What would be the best output to look at for this purpose, i think its going to be S11 LogMag or SWR ??

Also if i was looking at say S11 Logmag results, what would be a value that tells me "this aerial is no good at that frequency" ??

Something like -10db or what??

Many thanks

Dave

David,
with NanoVNA you can not find out, if an antenna (think you called it 'aerial') is goo for your
purpose to receive signals, or not. Depends on your receiver, if it has an input impedance of
50 ohms, you could check, if your antenna has a good SWR or not. But SWR is only a matter
to match input (output) plus cable to the 'other side' = antenna. A good SWR says nothing
about the efficiency of an antenna. A 50 ohm resistor at the end of your coax cable will show
you an SWR of 1:1. This will be a perfect reading of S11, but that's not a good antenna.

To bring it to the point: nanoVNA is not a tool to let you know, if you are having a good and
efficient antenna for your existing receiver and the frequency you want to listen at all..

I'm in the UK but i can use antenna as well, same thing ;)

But the Nano can show the resonant frequencies of an antenna and that is fairly pertinent to how it would perform ?

Not necessarily. A dummy load will have a good match - maybe even perfect - but does not make for a good antenna.

David,

yes, SWR (resonance) is fairly pertinent for an antenna. But once again, this does not
say, if this is a good antenna or not. Efficiency = good antenna is a matter of antenna
gain and resonance is only a small portion of it. A resonant short wave antenna in 1m
above ground will bring different results than one in resonance and 20m above. Both
of them will show a perfect SWR of 1:1 and other S-parameters, but the outcome ?

nanoVNA can only measure some technical parameters of your antenna - and they say
nothing about 'how good is this antenna' for receiving only ;-(

Ok, thanks

I'll focus on transmitting then as it generally goes that an antenna thats good for transmitting on a certain band will also receive on that band ;)

On Thu, 26 Sep 2019 at 15:10, Wolfgang OE1MWW <oe1mww@...> wrote:

David,

yes, SWR (resonance) is fairly pertinent for an antenna. But once again,
this does not
say, if this is a good antenna or not.

SWR and resonance are not the same thing. An antenna can be resonate but
have a VSWR of 100:1. Another antenna may have a 1.5:1 VSWR, but not be
resonate.


Efficiency = good antenna is a matter of antenna

gain and resonance is only a small portion of it.

Efficiency of an antenna has nothing whatsoever to do with resonance. A
lossless dipole of 0.5 wavelengths long has an impedance of around 73 + j
42 ohms, so is not resonate. However, the dipole is 100% efficient as it is
lossless.

The efficiency of an antenna is

(Power radiated) / (Poor accepted by the antenna)

If you have an antenna with a very high SWR, it will not radiate much. But
neither will it absorb much power.

Note also that an antenna with an impedance of 75 ohms will have a VSWR of
1.5:1 in a 50 ohm system, but 1:1 in a 75 ohm system.

I base my above comments on
145-2013 - IEEE Standard for Definitions of Terms for Antennas



You can get a copy without payment using sci-hub.


G8WRB.
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom

On Thu, 26 Sep 2019 at 16:35, Dr. David Kirkby <
drkirkby@...> wrote:

SWR and resonance are not the same thing. An antenna can be resonate but
have a VSWR of 100:1. Another antenna may have a 1.5:1 VSWR, but not be
resonate.

What I meant to say is that changing the system impedance doesn’t change
the efficiency of the antenna. You change the efficiency by changing its
losses.

Dave


--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom