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

[Andy Talbot <andy.g4jnt@gmail.com>]

Nope, the loading coil was  outside and fed with 15m or so of coax.   The issue I found was that there were two tuning points that had a purely resistive value.  But the two real values of R were very far apart; the wanted one (about 100 ohms at 137, equal to mostly ground losses) and another with very high R caused by an L match transformation.

It comes about like this.  Start with the loading coil correctly tuned to give the 100R AT IT'S BASE.  Add a tiny bit more L, then apply the shunt c of the short feeder.  We now have a series L shunt C network and if the bit of extra L is the correct value it will transform to a higher value of R

I found it, by testing with a small auxiliary coil indoors but that coil wasn't teh reason

Andy

www.g4jnt.com

On Tue, 30 Oct 2018 at 10:29, Alan Melia <alan.melia@btinternet.com> wrote:

Ok Luis I remembered after I posted, that I think Andy's loading coil was indoors fed by short coax run to the wire. So the capacitance of the coax was across the link, or tapped section. Pure water is an odd substance it was a very high dielectric constant .....around 80 from memory.

 

Alan

----- Original Message -----

From: VIGILANT Luis Fernández

To: rsgb_lf_group@blacksheep.org

Sent: Tuesday, October 30, 2018 9:53 AM

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

Hi Alan, LF

 

Thank you for your comments

 

Yes, the coil is at the antenna, at the end of the 30m coax line. Attached is a sweep between 8 and 300Khz

There is only one resonance at 137KHz as you can see

 

73 de Luis

EA5DOM

 

De: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] En nombre de Alan Melia
Enviado el: lunes, 29 de octubre de 2018 20:15
Para: rsgb_lf_group@blacksheep.org
Asunto: Re: Antenna resistance (Re: LF: EbNaut Autodecoder 137490 Hz: EA5DOM received)

 

Luis you mention measuring at the end of 30m of coax. Presumably the loading coil is at the antenna?? In which case you need to check that you have not parasitic resonances at harmonics of the drive frequency. Andy G4JNT reported some problems with that kind of set-up but shorter length of coax. Maybe you do have the coax matched?? I am not sure but I suspect your VNA may have a wide band detector which would respond to harmonic power?? I suppose a sweep at 270kHz would check this ?

 

My measurements of antenna+loss resistance were made with the antenna unloaded (not tuned) The equivalenct should be a series resistance and capacitance. Finbar EI0CF made many measurements on our inductively top loaded system. (see http://g3nyk.ham-radio-op.net/spiraltop.htm ) We found that the resistance increased during rain but recovered soon after it ceased. I was never able to determine what the cause was. It was unlikely to be increased insulator loss at the low bridge drive levels. I did think that rain water would be high resistance, and until it had time to disolve salts from the soil the ground resistivity would be high. Of course it never rained when we wanted to test this but a gale destroyed the system.The spiral top-load catches a lot of wind!   

 

Alan

G3NYK

----- Original Message -----

From: VIGILANT Luis Fernández

To: rsgb_lf_group@blacksheep.org

Sent: Monday, October 29, 2018 5:34 PM

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

 

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 28^th 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

 

De: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] En nombre de DK7FC
Enviado el: domingo, 28 de octubre de 2018 16:26
Para: rsgb_lf_group@blacksheep.org
Asunto: Re: Antenna resistance (Re: LF: EbNaut Autodecoder 137490 Hz: EA5DOM received)

 

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

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.