Re: phase angle and effective impedance

On Sep 17, 8:32 am, "Bob Martin" <bmar...@xxxxxxx> wrote:
"Brian Reay" <s...@xxxxxxxxxxxxxxx> wrote in message

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"Bob Martin" <bmar...@xxxxxxx> wrote in message
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Jim, I'm now not clear on where you are measuring with the VNA.

I measured first at the base of the antenna with a short 50ohm coax lead
from the coil tap and earth to the VNA (no transformer)

Xs was 5
Rs was 88 ohm
phase angle was 15 degrees
Impedance was 78 ohm and SWR 2.1

Back at the shack at the end of the 40m or so coax the Xs was 25
Phase was 30 degrees and impedance was 95 ohm for a SWR of 2.5

With the transformer  in circuitat the antenna before the vNA, the Rs
went down to 55 ohms and the impedance was around 48 ohms giving a SWR of
1.3 and at the shack end with the transformer on the end of the coax just
before the mini VNa it gave similar numbers

Rig into resistive DL gives 70 watts.  Rig into resitive DL with SWR
meter and RF ammeter in series give similar power readings within 10% or
so (quite good actually I thought!)

Rig into ant with 1.3 SWR as per VNA gives 25watts and current to match

SWR meter if in circuit reads 5!

Confused!  I am!

Not so much now, I think.

It seems you were first comparing the SWR at the antenna (or computing it)
with that at the tx, which was connected to the antenna via your matching
network, coax etc.  In those circumstances, I'd not expect the readings to
agree (other than in the perfect match condition). It was this which made
me suspect a sensing issue but, given your measurements weren't taken in
the same place, that may have been a red herring (at least in part).

It seems your tx is good for 70W into 50ohm. Assuming it is nominally a
100W tx, the "lost" 30W could be fold back (due to sensing) or simply due
to being operated outside its design range (John's point).  Which it is,
isn't so easy to determine- you could inhibit the sensing circuitry but
I'd be tempted to live with it. I doubt the duty cycle will mean the tx
suffers.

So, we are left with the matching issue. I favour matching at the antenna
end (although at 500kHz even with 40m of coax the advantages are not what
they are are higher frequencies).

You mention a "coil tap"on the antenna, can you not get a good match (as
measured at the tx end of the the coax) by adjusting that tap? That would
avoid any (extra) transformer and the associated losses.

Also, at 500kHz, I'd sort of think of a variometer to match the impedance
at the antenna base- although maybe you've good reasons for not taking
that route.

If you are wedded to a transformer (and given the low reactance of the
antenna it may work), I'd still place it at the antenna end and adjust for
best match as measured at the tx end of the the coax.

Your tuning bandwidth requirement should mean you don't need to "retune"
your antenna and thus remote matching isn't an issue.

--
73
Brian G8OSN/W8OSN
www.g8osn.net

Thanks for the input here.  I'll try the transformer at the antenna end and
the measure with the VNA at the shack end.  (the exercise will do me good!)

I think I need more coils on the transformer too, keeping the ratio the
same.

Best dx so far on 500 is 960km to Sweden using WSPR and worked a few
inter-UK on CW 12wpm.   The big guns on 500 routinely do 1500km plus and
have been transatlantic and even China on WSPR so thats the next objective,
but I need to get my antenna a lot better first and stop warming the grass!

cheers

Jim- Hide quoted text -

- Show quoted text -

Don't rely on the MiniVNA being accurate enough to perform the
measurement.

They don't resolve the phase angle acurately and are very prone to
interference from broadcast stations when connected to an antenna.

See if you can get hold of an AIM 4170

UKM
.

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