On Fri, 20 Sep 2019 at 18:33, Ken Buscho <kb6kob@...> wrote:
Hi, Folks,
My whole NanoVNA process started because I went out on eBay looking for an
antenna analyzer for ham VHF/UHF use and got my white model because it
intrigued me at what was being done at that price point. No prior personal
experience with VNAs, although I've been aware of them for years. Big
learning curve to get started, but it did almost everything I was looking
for once I could see SWR and a Smith Chart. It's been well worth the $60,
and I'm getting a VNA education to boot reading here.
I say almost because I would like to be able to get some values to at
least compare HT Antenna A to HT Antenna B for gain. To get started, I put
a test antenna on CH0 and a reference antenna on CH1. I can tell the CH1
antenna is "seeing" the CH0 antenna because logmag dB values change
with/without something on CH0, and I can also see that dB values go up/down
as antenna distances change.
What I'm looking for is suggestions on how to do this the right way, or at
least a better way. . Suggestions? Issues?
Appreciate the input...
Ken Buscho
KB6KOB
I don¡¯t think you will manage just using the NanoVNA. First understand that
as hams we are interested in the far field performance. If you are working
between a few hundred kHz, the near field maybe important, but generally
you are only interested in the far field properties.
There are two ways to measure the far field gain of an antenna.
1) *Far field* measurements usually made on an antenna test range. Only the
amplitude of signals need to be measured. Amateurs can do this.
2) *Near field *measurements made in an anechoic chamber measuring both
amplitude and phase. The receiving probe needs to scan over the whole
antenna. Amplitude and phase need to be measured, so a VNA or similar is
essential. Once you have the near field pattern, the far field can be
computed by a Fourier Transform of the near field. This is totally out of
the question for hams to do - it needs very specialist equipment.
*So you need to make far-field measurements. *
In order to make far field measurements, the antennas need to be
sufficiently far apart - a distance R of
R = 2 D^2 / lambda
where D is width of the antenna.
Assuming isotopic radiators, the loss is called the free space path loss
(FSPL)
What I believe you will find is that the free space path loss will be too
high to measure with the NanoVNA. The dynamic range of the NanoVNA will not
be high enough. I suggest that you work out the distance to the far field,
then work out the attenuation in dB to check this. Adding a power amplifier
after calibration could help, but you will still probably be unable to do
it.
A more practical way will be the power output from an amateur transceiver
and a calibrated power meter, which could be a receiver. *Much* greater
attenuations can be measured this way.
I am 99.9% sure that the dynamic range of the NanoVNA will be insufficient.
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
