Impedance matching with SID monitorong

[Andrew Thornett]

Hi All

Does impedance matching on VLF antenna used with SID monitor to the impedance on the receiver matter? Hence if number turns of coated wire on the antenna cross increased will that adversely affect the performance of the system due to impedance mismatch?

Andy

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[Stephen Arbogast]

Yes it  is very   important  .....    I  am more  Physics  than    EE     I use  return   loss     that  is   convertible   to   SWR   If  you   don't have  an  impedance   match       you  will have   standing waves on    your  feed

 line ....    that leads  to  losses    in the   coax...     both  transmitting    and  receiving     the   reciprocity   theorem.....

[Alex P]

ANOTHER LOOK AT REFLECTIONS
By: M. Walter Maxwell, W2DU/W8HKK

https://www.k3emd.com/downloads/Reflect.pdf

[b alex pettit jr]

Better Link 

https://old.w1yca.org/tech/ant/reflectIII.pdf

========================================================================

A Quote from amateur radio operator ON5HB

"    The best book ever to understand how an antenna-system works.
SWR is not important. What matters is low attenuation of the transmission-line.  As SWR is simply power returning down the cable and will face attenuation more then once.  As such attenuation multiplies. Example, if you have a 3dB att of your cable, at 1:1 from 100W only 50W reaches the antenna. But when SWR is high and 25W is returned, it means that 50W-25W-6W will get into the antenna...ergo out of 100W, just 31W. The rest is gone. When you do the same with a cable that has 0dB losses (best cable there is) then 100W-50W+50W will be transmitted, ergo full 100W.  SWR matters only on high-loss lines, it doesn't matter on low-loss.  That is what his books teach you....and he is right. I do not care about 10:1, as my cable is just 0.2dB loss at 30m length.  The losses are very low, even at high SWR. He teaches us that poor cables/lines will kill your power at the antenna.....yeah and also at 1:1.   "

[Douglas Decker]

Hi Andy and others. here goes my 2 cents again.

I have been doing SuperSid now for sometime.(Time is relative)

I am sure that on a transmitter SWR is very important.

But on Receivers not so.

I have never tried to measure vswr on a receive only antenna,  except once, on my "HL" dish, and that was to make sure the antenna was as close as possible to the signal I was looking for.

My ham radio antennas ie like 40 meters is tuned to 7.160mhz  with minimum vswr at transmit.

I can scan the entire 40 meter band with the receiver and can not tell any difference on receive signal levels. 

But when I transmit,  I always have to check and retune the antenna to minimize the   vswr so as to reduce the reflections into the final rf stages

to prevent overheating(IE Reflected power).

 So back to VLF if you are doing it for Solar Flare detection

 PS great resource is the SuperSID email group:

( https://groups.io/g/supersid  )

I am a new user also to VLF and Solar flare detection.

I have two loops.

One is 2 foot square and the other is about 4 foot square.

In the documentation from Stanford they recommended about 400 feet of wire.

So both loops have 400 feet plus or minus the amount need to complete a loop.

I have tried both with out the SuperSid receiver\preamp and with.

The biggest difference has been in amplitude. Very little difference noticed.

The preamp provides some level of protection of the soundcard input and some gain. That's it.

Plus it has a serial number used by Stanford to identify your unit for the FTP uploads to Stanford.

The display that the SuperSid software shows is a spectrum display based on the band width of the soundcard.

So when you set up SuperSID in the config file you give it a list of the frequencies you want to catch and store.

 

So when I first got started I did some testing to see what signals I was receiving.

I rotated the antenna, looked at the published list to see which one I could see.

Then I put the best ones in the config file.

 

All the other signals that show up that are not identified are noise, or interference or something else. Nothing I am interested in.

I thought about building a selective bandpass filter but then the brain said what for.

I only look at the ones I want to and the software ignores the rest. 

 

The best part is: I get to share data with others via Stanford FTP Servers. I also can 

go and download others data from their site and compare to mine.

What a novel concept to share and compare.

We should try the above on other RA areas as well.

 

I hope my rantings bring a little bit of positive reinforcement to  others.

I did not have to be a scientist to do this just another person interested in the same thing as others.

 

Best of Luck Doug K5WMT.

[Jeremy Waller]

Greetings to all,

It has been interesting following this thread.

 I have been doing work in the area of impedance matching in relation to my 2 band (20m and 40m) vertical antenna. I have designed the matching using both lumped constant components(L's and C's) and using a transmission line as a transformer. These designs were done were done from first principles and I am in the process of writing up my work using transmission lines as transformers. 

Re:  I am sure that on a transmitter SWR is very important.

But on Receivers not so.

Antenna matching is very important in both reception and transmission. To understand this one needs to model the circuit from the point of view of both the transmitter and receiver. To do this one must understand the application of the "Maximum Power Theorem" and creation of the equivalent circuit using Thevenins Theorem.

When the match is perfect one gets maximum power transfer "for free". This can be shown but the maths looks "untidy".

Here is a figure (figure_6.jpg) from my write-up. The two paths( Rx and Tx) are shown.

Re: I have never tried to measure vswr on a receive only antenna

This can be done. But to get meaningful results it takes a great deal of care. This signal levels being low and with the noise sources in play (Receiver noise and Environmental noise, mercifully being uncorrelated!!) one will need do these measurements from a point of view of input and output SNR's. Please remember that one noise source is affected by the antenna matching where as the other is not affected.

73 to you all,

Jeremy.

[Jeremy Waller]

To follow up on the last post, here are some calculations and measurements to illustrate what happens on the receive and transmit paths of the matching line transformer for the vertical antenna operating in the 40m band and the corresponding impedance presented to the input of the receiver.  Please note the change in antenna over the 40m band and the corresponding out put impedance at the end of the transmission line match. It looks as if I need to calculate the proportion of energy received by the antenna that is passed to the receiver input ... I will include this in my write up!! The transformer transforms the antenna impedance into a conjugate match for the receiver.

In the second part - the transmit path - the impedance presented by the transmitter  to the tx line transformer is called the line input. At the centre frequency the tx line presents a conjugate match to the antenna. When one calculates the output power  notice that the power output is higher than the max power... but observe the power factor  (cos(phase_angle)).

Sorry to be so long winded but matching is most important on both and receive paths.

73

Jeremy

For the receive path note the change in the antenna impedance at the 5 frequencies

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[Nathan]

With the wavelengths so long at VLF frequencies, I would think that a
transmission-line treatment is not necessary.  Instead, regard the loop
as a lumped inductance with a resistance.  The voltage induced by the
transmitter's magnetic field generates a current that must pass through
the loop inductance and wire resistance to reach the receiver input. 
Note that both the loop's induced voltage and the wire resistance scale
with turn count, while the loop inductance scales with turns squared. 
One can then ask how the received signal intensity varies with turn
count and solve via circuit analysis.

When I modeled the loop resistance as 0.01 ohm per turn, the inductance
as 4 μH times turns squared, the receiver input impedance as 600 ohm
resistive, and frequency as 25 kHz, I get the attached plot for a
relative gain as a function of turn count.

Perhaps the greatest uncertainty of this calculation is that there may
be resonances in the loop from capacitive coupling among turns.

Nathan

[Steve Berl]

That seems pretty close to the numbers in https://solar-center.stanford.edu/SID/DOC/SuperSID-Manual.pdf

See page 13.

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-steve

[Andrew Thornett]

That's interesting- so basically the benefit of extra turns only goes so far then drops off rapidly

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From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Steve Berl <stev...@gmail.com>
Sent: Monday, October 20, 2025 8:16:41 PM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: Re: [SARA] Re: Impedance matching with SID monitorong

 

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[Nathan]

To take this antenna discussion one step further, one can get a feel for the loop-size dependence of this gain factor as predicted model.  Note that the induced voltage is proportional to the area of the loop, and that the resistance and inductance are both proportional to the linear size of the loop (in the latter, ignoring a weaker log factor).  Then solving for the gain with the parameters as before, I get the attached graph for loop sizes of 1, 2, and 3 meters diameter, traces from bottom to top.

Nathan

On 10/20/25 14:08, 'Andrew Thornett' via Society of Amateur Radio Astronomers wrote:

[Curt Kinghorn]

Whit Reeve, long time and very active SARA member, gave a presentation at one of SARA’s Western Conferences where he showed with actual data that impedance matching wasn’t as important as one might think. I tried finding his presentation but couldn’t find it. I suggest you reach out to him about this.

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[Marko Cebokli]

Hello!

Check the "electronics unmessed" channel on youtube, he made a very nice single turn loop for VLF, using a simple transformer for impedance matching. There are several videos, explaining the theory and practice of making one. If you just listen on the 20kHz band, you do not need the complex band switching arrangement he developed, a single secondary will do.

Marko Cebokli

2025-10-22 21:43, je Curt Kinghorn napisal

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[Whitham Reeve]

Good to hear from you, Kurt!

I made a presentation at the Western Conference in 2010 on the VLF receiver sold by UKRAA. The presentation was based on the paper here: 

https://www.reeve.com/Documents/LF-VLF/Reeve_SARA%20WestConf%20VLF%20Paper_Upd.pdf

I also did a two-part presentation in 2019 on Monitoring Low Frequency Propagation with a Software Defined Radio Receiver. The associated papers and presentation are here:

https://www.reeve.com/Documents/Articles%20Papers/Propagation/Reeve_LFProp-ConceptsP1.pdf

https://www.reeve.com/Documents/Articles%20Papers/Propagation/Reeve_LFProp-ObsvP2.pdf

https://www.reeve.com/Documents/Articles%20Papers/Propagation/Reeve_VLF_2019_SARA.pdf

You can find a complete set of papers on VLF & LF applications and antennas here:

https://www.reeve.com/RadioScience/Radio%20Astronomy%20Publications/Articles_Papers.htm#Very_Low_Frequency_Applications

Whit

[Stephen Arbogast]

Find  transmission   line  theory   here..     https://en.wikipedia.org/wiki/Transmission_line

It is   very  important and my experience that   impedance  must be matched    at both ends of  the  transmission  line.  I have  worked mostly in   VHF and  above. 

Remember  paper Smith  Charts and hand  held calculators from  the early1970's?

[Stephen Arbogast]

Keep in mind  that  an antenna is not  a  circuit  element.....   

[Stephen Arbogast]

https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem