Re: Antenna resistance (Re: LF: EbNaut Autodecoder 137490 Hz: EA5DOM received)

[Markus Vester <markusvester@aol.com>]

> Something gets wet in the basement and the pipe gets a better connection to ground, or at least to the water grid pipes, which finally run underground

Ah that must be the reason! That drain pipe is obviously not continuous metal but inrterropted by some insulating joints or plastic sections, which are more or less bypassed by a water film or flow when it rains.

Could you try to use another ground connection which is continuous all the way down? E.g. external lightning protection conductors, or even the protective earth in your appartment?

Best 73,
Markus

-----Ursprüngliche Mitteilung-----
Von: VIGILANT Luis Fernández <luis@vigilant.es>
An: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Verschickt: Sa, 27. Okt 2018 23:17
Betreff: Re: Antenna resistance (Re: LF: EbNaut Autodecoder 137490 Hz: EA5DOM received)

Hi All, LF

Interesting topic. Also happens in MF but in LF it gets more critical. Must be a nightmare at VLF :(

As Marco explained, our setup is not the usual at ground level in a property plot. And in my case ther ground

is 80m away down from the feed point, coil and capacitive topload.

Measured with the antenna analyzer I can see a drop in the impedance module when all gets wet by rain

It is not only an increase of RF current. Today I measured resistance from the ground side of the feed point

trought the pipe to building basement and then returning via the water grid connecting the other side of the

multimeter to a water pipe in my house:

4 Ohm measured early morning when it started with soft rain. At noon we had stronger rain and in the afternoon

I measured again. It dropped to 2 Ohm. Big difference. Something gets wet in the basement and the pipe gets

a better connection to ground, or at least to the water grid pipes, which finally run underground

The top of the antenna is dismantled during storms, so can't measure impedance with the analyzer right now

but bet it is now well under 30 Ohm and would drain a few amps

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. 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. 

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

https://www.dropbox.com/s/ejn5stuz7vban7u/Ropex%20Schematic.JPG?dl=0

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

73 de Luis

EA5DOM

---

De: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] en nombre de marcocadeddu@tin.it [marcocadeddu@tin.it]
Enviado: sábado, 27 de octubre de 2018 18:12
Para: rsgb_lf_group@blacksheep.org
Asunto: R: Antenna resistance (Re: LF: EbNaut Autodecoder 137490 Hz: EA5DOM received)

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: paul@n1bug.com
Data: 27-ott-2018 12.34
A: <rsgb_lf_group@blacksheep.org>
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.