Well, let's say that in most times one port VIA would be enough, to tune an
antenna basically, and some other minor jobs.
But some times you need to phase a stack of Yagis, or even a four-square
verticals for low bands, and two ports VNA allows you to fine tune the phasing
lines more accurately than one port VIA. To measure iteration/isolation between
antennas, or relay boxes, etc.
You can even run some real gain and front to back measurements, using one port
to a reference antenna and second port to the antenna under test.
Finally, if you can afford to have different devices for different uses, that's
perfect, a light handheld VIA for antenna tuning and tower works, and a second
two ports VNA for bench works, for example.
But if for any reason (field day, expeditions, someone's else house, limited
budget...), you need, or you prefer, just one device for all situations,
something like a "Swiss Knife", a VNA like this KC901S is best choice, as far
as I can see.
73, Maximo
________________________________
De: TowerTalk <towertalk-bounces@contesting.com> en nombre de Jim Brown
<jim@audiosystemsgroup.com>
Enviado: jueves, 6 de diciembre de 2018 18:43
Para: towertalk@contesting.com
Asunto: Re: [TowerTalk] Antenna Analyzers
On 12/6/2018 2:36 AM, Máximo EA1DDO_HK1H wrote:
> Most ham devices cover HF and some of them VHF. Good for a majority of users.
> For those with UHF needs... best handheld VNA I know (not professional
> devices) is the KC901V or KC901S+.
In this discussion, it is important to understand the difference between
a VNA, which is a two-port device, and a VIA (vector IMPEDANCE
analyzer), which is a one port device. If it's got only one connector on
it, it's a VIA. A VIA measures S11, which can be used to compute the
impedance, and a suitable sweep can be post-processed to do TDR. A VNA
adds the capability to measure S21, the GAIN between its input and
output. Maybe I'm dense, but I can't think of a reason to need a
handheld VNA, or to need one on a tower.
S21 measurements are typically used to measure gain of electronic
systems, the response of bandpass filters, including systems with
cavities, and the isolation between antennas. I also use S21 to measure
high impedances, like common mode chokes and small values of
capacitance. For the latter measurement, a specially constructed test
jig is required.
Most antenna analyzers are single-port devices, and the best of those
are VIAs. This includes all of the Rig Expert units, the SARK, the
FA-VA5, and all of the single-port AIM units.
73, Jim K9YC
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