Hi Stefan, LF
> How do you measure the antena current?
Are you sure the waveform still looks like a sine?
I have an old thermocouple Amp meter, 4A range. Couldn?t check if
calibration is correct but better than nothing
Yes, the output waveform from PA is a perfect sine in the
scope
Hm, are you sure your antenna has 50 Ohm
when it is resonated?
With the analyzer I can measure many parameters. Module of
impedance and also ancge of impedance, which cross zero
at the same point were SWR is minimum.
Attached two charts corresponding to measures made on Saturday and
Sunday. You can see that antenna is resonating at about
138.500
and the cursors are at 137.511 and 138.078. In october
28th the impedance is about 100 Ohm at resonance and next day it
dropped to 32 Ohm
We had some rain during weekend that caused this change in the
impedance
I?m not measuring the antenna at feed point but via 30m RG58 coax.
The analyzer is compensating for this cable anyway
Otherwise you may need a transformer to
transform it to 50 Ohm. Or your LPS is not working properly
Tried a short transmission without LPF. Same results. No
difference
What power output do you get when using a
50 Ohm dummy load? And what efficiency?
Last time I checked this I was measuring 72Vrms over 50 Ohm dummy
load, about 100W
Didn?t save the current measurements at 12V. But with 24V I got
114Vrms over dummy load and the PA current was 16A@24V
73 de Luis
EA5DOM
Hi Luis,
Am 27.10.2018 23:11,
schrieb VIGILANT Luis Fernández:
Hi All,
LF
Interesting topic. Also happens in MF but in LF it gets more
critical. Must be a nightmare at VLF
:(
Here it is more critcal on MF than on
LF. At VLF it is quite uncritical because the coil's resistance is a major
part of the overall losses.
Other
intringuin detail. When it is dry and impedance rises to >100 Ohm the
only way to hardly reach 1A RF
is to
detune the antenna. I have to set the resonant peak at 138Khz and then at
137,5 get even higher impedance
and
high SWR, but the antenna current rises and so the radiated
signal.
How do you measure the antena current? Are
you sure the waveform still looks like a sine?
All
this at the cost of more amps
consumed by the PA and higer voltage at antenna feed. Running the
antenna at the optimum resonance makes
almost
no RF current, even considering that this is the minimum impedance point.
Hm, are you sure your antenna has 50 Ohm
when it is resonated? Otherwise you may need a transformer to transform it to
50 Ohm. Or your LPS is not working properly. What power output do you get when
using a 50 Ohm dummy load? And what efficiency?
73,
Stefan
About
the LPF, this is really the only part of the Ropex PA still in use. This is
a link to the schematic with the LPF
Have
not tried without LPF. The analyzer measurements are made directly to
antenna. But may be a good idea to
make
them with the LPF inserted and see what happens
Hi Paul,
Luis, Markus, LF
let me join the topic ;-)
the antenna location of
Luis is something else of your Markus and Paul and much similar to mine:
roof based antennas with elevated ground (in my case) and vertical ground
(the water pipe) for Luis.
Anyway also in my case the serie resistance of
my antenna increases with rain (almost doubles).
But.. Luis, probably
assumes the lower impedance basing on the higher current.
Luis, what kind
of LPF have you adopted?
Now I have a T filter with no shunting cap at
the input and impedance increases with rain both with and without LPF, but
if I remember correctly, when in the past I had an PI LPF with shunting
input and output caps the impedance in my shack was transformed from the
transmission line and LPF giving lower impedance by rain.
Could be an
explication?
By the way, just for fate it's raining also here and I
made some measurement when dry again will repeat ;-)
Have a funny
weekend LF
Marco IK1HSS
----Messaggio
originale----
Da: [email protected]
Data: 27-ott-2018
12.34
A: <[email protected]>
Ogg: Antenna resistance (Re: LF: EbNaut Autodecoder 137490
Hz: EA5DOM received)
Hi Luis, Markus, LF
This is a very
interesting topic for me too.
My antenna resistance is very high,
usually more than 100 ohms. I
think there may be a few reasons. There are
trees near the antenna
which I presume to be lossy. My antenna ground
system is not tied
into the house ground, in order to minimize 120 Hz
sidebands on my
transmitted signal. When I tie the grounds together the
level of
these sidebands increases almost 20 dB. Also my RX antenna is
more
noisy when the TX antenna ground and house ground are tied
together.
I suppose this is because more noise is radiated from the
TX
antenna, then picked up by the RX antenna which has its own
small,
isolated ground.
Rain has no effect on the antenna
resistance, but temperature does.
During summer there is little change,
just minor variations. But as
the temperature drops and begins to
approach freezing, resistance
begins to get lower. There is a big drop
right around freezing or
few degrees below, but resistance continues to
drop with even lower
temperatures, reaching minimum on the coldest winter
nights. Last
winter I saw it reach 40 ohms a few times. :)
I am no
expert but I guess the trees are partly responsible for the
high
resistance and the drop with freezing temperature?
Anyway I can run
enough power to reach calculated 1W EIRP and it's
enough to be heard
across that little pond. :)
But here is a related question: I have
calculated my EIRP to be one
watt using the measured resistance, antenna
parameters and current.
But does this calculation include all the losses
in trees? If I had
the proper equipment to make field strength
measurements at a
distance, would I find that my EIRP is less than one
watt due to the
tree losses?
73,
Paul N1BUG
On
10/27/18 5:45 AM, Markus Vester wrote:
> Hi Luis,
>
>>
Impedance drops a lot after rain
>
> This seems unusual. I have
exactly the opposite effect here:
> Series resistance at 137 kHz is
normally around 20 ohms
> (including 5 ohms for the coil). When it
rains it get worse up to
> about 40 ohms, whereas best times are cold
and dry winter days
> with ~15 ohms when the trees are solidly frozen.
So I presume
> that the major contribution to my resistance
are
> capacitively-coupled losses in vegetation and other
nearfield
> environment. The ground connection itself (the "house
earth"
> which is connected to electric grid PE, and also pipe systems
for
> water supply and distict heating) seems to have a low
resistance.
>
>
> My only explanation for lower wet
resistance would be if one were
> using a separate radial system in
the garden, whose connection to
> ground might be better when the lawn
is soaked. But I don't think
> this applies to your setup at
all.
>
> Best 73, Markus
>
> Von: VIGILANT Luis
Fernández
>
> I'm struggling
> here with the antenna.
Most of the time impedance rises and I
> hardly run 1A RF
current
>
> Rainy weekend here.
> Impedance drops a lot
after rain. Let's see how much current can
> drainLast time I could
see almost 4A
RF.
