Original Message -----
Monday, October 29, 2018 5:34 PM
RE: Antenna resistance (Re: LF: EbNaut Autodecoder 137490 Hz: EA5DOM
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
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
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
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
I’m not measuring the antenna at feed point but via 30m RG58
coax. The analyzer is compensating for this cable
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
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 [email protected]
73 de Luis
Am 27.10.2018 23:11,
schrieb VIGILANT Luis Fernández:
Interesting topic. Also happens in MF but in LF it gets more
critical. Must be a nightmare at VLF
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.
intringuin detail. When it is dry and impedance rises to >100 Ohm the
only way to hardly reach 1A RF
detune the antenna. I have to set the resonant peak at 138Khz and then at
137,5 get even higher impedance
high SWR, but the antenna current rises and so the radiated
How do you
measure the antena current? Are you sure the waveform still looks like a
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
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
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
not tried without LPF. The analyzer measurements are made directly to
antenna. But may be a good idea to
them with the LPF inserted and see what
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).
Luis, probably assumes the lower impedance basing on the higher
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
Da: [email protected]
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
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
high resistance and the drop with freezing
Anyway I can run enough power to reach calculated 1W
EIRP and it's
enough to be heard across that little pond. :)
here is a related question: I have calculated my EIRP to be one
using the measured resistance, antenna parameters and current.
this calculation include all the losses in trees? If I had
equipment to make field strength measurements at a
distance, would I
find that my EIRP is less than one watt due to the
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
> with ~15 ohms when the trees are solidly frozen. So I
> that the major contribution to my resistance are
capacitively-coupled losses in vegetation and other nearfield
environment. The ground connection itself (the "house earth"
is connected to electric grid PE, and also pipe systems for
supply and distict heating) seems to have a low resistance.
> My only explanation for lower wet resistance would be if
> using a separate radial system in the garden, whose
> 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