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The mysteries of tuning a random wire


Dave W0DCX
 

Here is another antenna tuning conundrum.  This morning I hoisted up a 72-foot random wire (and deployed a counterpoise).  My calculations suggested it should tune great on 30 meters (almost exactly 3/4 wavelength) but poorly on 40 or 20 meters (close to 1/2 wave multiples).  Here is what I found in terms of SWR of the untuned antenna as determined by my NanoVNA and what I observed when I tried to achieve a good SWR match with my KX2 automatic antenna tuner.

30 meters - VNA SWR = 2.3; KX2 can tune to SWR = 1.0  : NICE!
20 meters - VNA SWR = 13.17; KX2 can tune to SWR = 1.0 : NICE BUT SURPRISED!
40 meters - VNA SWR = 4.4; KX2 can only tune to SWR = 4.4 : TOTALLY BAFFLED - WHY?

Any insights?
Dave W0DCX


Jim, N5IB
 

Go a step further with your Nano.

After getting the swr data, also get the impedance information (R +/- jX). You may find one case of high swr involves a fairly low impedance, while another case is associated with a fairly high impedance. Some tuners are happier matching one or the other. depending on their design. Especially if there is a large reactive (X) component of the impedance.

Report back what you find, and also report:
is there a feedline involved, or do the counterpoise and antenna both terminate at the rig?
how high is the far end of the antenna?
is the counterpoise a single conductor? how long?
how is the counterpoise deployed? on the ground? elevated? if so, how high?

N5IB


Dave W0DCX
 

Jim, thanks for your mentoring.  I am going to follow up and report back with the specific data you requested.  What I am trying to do here is explore the electromagnetic behavior of random wires AND try to optimize performance of the simplest and most efficient designs possible.  So my goals are both academic and pragmatic.  My usually practice is to use a 9:1 unun to feed my random wires.  In combination with the KX2 ATU this makes almost all things possible.  But I wanted to reduce/simplify/streamline as much as possible the electronics involved so I am now going back and experimenting with random wires fed directly by the transceiver without the unun.  The idea, perhaps an erroneous one, is maximize power efficiency in the output circuit by providing a direct connection between the XCVR ATU and the antenna - no unun and no coax.  Trying to get as much oomph from my QRP output as possible.  I might be on the right track if. and that is a big if, I can get the antenna and the counterpoise perfectly optimized for a specific frequency.  Is there any validity to my thinking? Or am I just spinning my wheels and worrying too much about a little power loss.

So, in answer to some of your cogent questions; 1) no feedline - antenna and counterpoise connect directly to XCVR ATU, 2) far end of antenna is at about 35 feet, 3) counterpoise is single conductor , 4) counterpoise runs down through floor and runs along top of basement foundation wall so about 18 inches above ground, and 5) counterpoise is only 17 feet long.  Okay, that seems rather short for a 40m band counterpoise so I will try lengthening that for 40m or better yet add a second counterpoise of 33 feet.
Thanks,
Dave W0DCX


J.K. Wright
 

You might enjoy this link:

Jerry, NK2C

On Sun, Jun 14, 2020, 12:24 PM Dave W0DCX <cwqrp73@...> wrote:
Jim, thanks for your mentoring.  I am going to follow up and report back with the specific data you requested.  What I am trying to do here is explore the electromagnetic behavior of random wires AND try to optimize performance of the simplest and most efficient designs possible.  So my goals are both academic and pragmatic.  My usually practice is to use a 9:1 unun to feed my random wires.  In combination with the KX2 ATU this makes almost all things possible.  But I wanted to reduce/simplify/streamline as much as possible the electronics involved so I am now going back and experimenting with random wires fed directly by the transceiver without the unun.  The idea, perhaps an erroneous one, is maximize power efficiency in the output circuit by providing a direct connection between the XCVR ATU and the antenna - no unun and no coax.  Trying to get as much oomph from my QRP output as possible.  I might be on the right track if. and that is a big if, I can get the antenna and the counterpoise perfectly optimized for a specific frequency.  Is there any validity to my thinking? Or am I just spinning my wheels and worrying too much about a little power loss.

So, in answer to some of your cogent questions; 1) no feedline - antenna and counterpoise connect directly to XCVR ATU, 2) far end of antenna is at about 35 feet, 3) counterpoise is single conductor , 4) counterpoise runs down through floor and runs along top of basement foundation wall so about 18 inches above ground, and 5) counterpoise is only 17 feet long.  Okay, that seems rather short for a 40m band counterpoise so I will try lengthening that for 40m or better yet add a second counterpoise of 33 feet.
Thanks,
Dave W0DCX


W4OED
 

As you know, long-wire antennas have a large inductive component. Some antenna couplers do not have the capacity to deal with such inductance and they rely on the operator to install the antenna coupler close to the radio. This way, the feedline should help it to deal with the extra inductance.
That is why antenna tuners for HF ship radios insert a capacitor in series after the tuning network when dealing with long-wire antennas. It is important to mention that such antenna tuners are always installed within three to six feet from the base of the antenna to avoid dealing with the feedline.
Regarding the counterpoise, I have seen great results when the counterpoise (about 1/4 wavelength of the lower transmit frequency) was installed a few inches about the ground, in line with the radiator, but OPPOSITE to it. This way, the counterpoise acts more as the other half of a dipole. It might be worth trying it.


Jim, N5IB
 

Now, no one ever made a QSO with a computer model of an antenna, but software like 4NEC2 can be useful to gain some insights.

Here's an attached simulated sweep from 80m to 10m of Dave's 72 ft wire. The model assumed:
72 ft long, 2mm diameter copper wire, sloping from 3 ft to 35 ft
17 ft counterpoise, same wire, sloping down from 3 ft to 1.5 ft
counterpoise in the same plane as, and in same direction as, the long wire
average ground conditions

Notice how the reactive part (red curve and axis scale) swings both inductive (+) and capacitive (-) with frequency. Notice also the reactance zero crossings (resonances) where the resistive part (blue curve and axis) is either at a peak, or an some intermediate value between the peaks.

The resonances where the resistive part is at a peak represent frequencies where the wire is behaving as an end-fed multiple of a half wave wire. The other resonances correspond to frequencies where the behavior is that of an **odd** multiple of a quarter wave, end-fed, and generally easier to match.

BTW --- 4NEC2 will calculate the L and C values of a matching network - Tee, Pi, or L (high or low-pass configuration) that will match a given feedpoint.

Dave, when you get a chance to do a sweep with your Nano, we'll all be curious as to whether the general appearance of your sweep is at all similar to this simulation.

N5IB


Dave W0DCX
 

I just read Jim's latest post.  Very nice.  I have not yet done a full sweep, but I did try characterizing this so-called antenna for a few bands.  I think the results are consistent with what I originally posted.  But I have to admit this test in probably flawed, for example I don't think my two counterpoises are fully and fairly comparable as they are each positioned differently.  But anyway, I did the work so here are the results.  One of the most marked features is that on 30 meters (the band that behaves the best) the reactive component X is inductive with the 17' cp but capacitive with the 29' cp!  All other reactances are always capacitive.  So 30 meters seems not to care too much either way, it tunes fine with my KX2.  Also, things seem to go south on 20 meters with a longer cp in terms of SWR.  Sorry about the piecemeal telling of this sordid tale.  I will do the full sweep as Jim suggested and see what happens.  I am still learning the ins and outs of the VNA.  Table is included as both a screenshot and a Word.doc attachment.
Dave W0DCX


Jim, N5IB
 

Hi again Dave,

Look and see if the Nano has an option to display the reactive part as ohms instead of capacitor/inductor units.
That'll make it much easier to interpret.

For example - in your 30 m measurements.... at 10 MHz, 402 nH is +25 ohms, while 572 pF is -27 ohms. So your 30 m results show that you are actually very close to one of those zero crossings. The change in counterpoise just shifts the zero crossing a little bit either way. That's not as apparent with nH and pF.

N5IB


Ronald Morrison
 

Highly recommended reading. Note recommendation about not connecting the counterpoise to the station ground.



Ron, K5DUZ




Jeff Logullo N0̸MII
 

Great find Ron — thanks for sharing!

W8JI’s site has a lot of good material. Found this page


which has advice on tuning T-match tuners for maximum efficiency. (Was that in this thread, or another?)


On Jun 17, 2020, at 2:44 AM, Ronald Morrison <rmrrisn@...> wrote:

Highly recommended reading. Note recommendation about not connecting the counterpoise to the station ground.


Ron, K5DUZ

--
Jeff N0̷MII


Dave W0DCX
 

Taking the next step I took N5IB's advice and took the impedance calculations to completion.  I'm not sure if the VNA will do this for me, but for now I constructed an Excel spreadsheet where I can just plug in the values the VNA gives me and get the final results.  I've checked the calculations a few times and don't find any errors.  The results still puzzle me.  Some of the numbers I can rationalize from the actual behavior of my ATU but then some of the numbers don't make sense.  I may be doing something wrong, or maybe there is some sort of voodoo going on in my wires.


Ronald Morrison
 

If you search for W4RNL you will find many great articles on antennas, tuners, etc.

Ron, K5DUZ

On Wednesday, June 17, 2020, 06:43:37 AM CDT, Jeff Logullo N0̸MII <jeff@...> wrote:


Great find Ron — thanks for sharing!

W8JI’s site has a lot of good material. Found this page


which has advice on tuning T-match tuners for maximum efficiency. (Was that in this thread, or another?)


On Jun 17, 2020, at 2:44 AM, Ronald Morrison <rmrrisn@...> wrote:

Highly recommended reading. Note recommendation about not connecting the counterpoise to the station ground.


Ron, K5DUZ

--
Jeff N0̷MII


Dave W0DCX
 

Sorry, some formatting inconsistencies in the table previously posted.  Here is the corrected version.


Don, W9EBK
 

Dave,
There are too many variables here for me to make a suggestion with much confidence.  I don't know what model VNA you are using but if it's a $50 mini-VNA it might be that the numbers you are getting are not accurate.  About a year ago there was an article in QEX where someone thoroughly tested one of those.  The results showed amazing accuracy most of the time,  but there were some situations where the numbers were Very wrong.  As I recall this was caused by measuring loads that were resonant on a harmonic of the test frequency.  The solution to this problem would be to use a bandpass filter for each band,  but in your case the filter might change the xL or xC of the device under test, so that's not likely to work. 

Don, W9EBK 

On Wed, Jun 17, 2020, 1:41 PM Dave W0DCX <cwqrp73@...> wrote:
Taking the next step I took N5IB's advice and took the impedance calculations to completion.  I'm not sure if the VNA will do this for me, but for now I constructed an Excel spreadsheet where I can just plug in the values the VNA gives me and get the final results.  I've checked the calculations a few times and don't find any errors.  The results still puzzle me.  Some of the numbers I can rationalize from the actual behavior of my ATU but then some of the numbers don't make sense.  I may be doing something wrong, or maybe there is some sort of voodoo going on in my wires.


Dave W0DCX
 

I want to thank everyone for their helpful and educational input.  I am going to study the resources provided and do some further experimentation with various random wire configurations.  .  If I discover anything of interest I will let you all know.  Best 73s!
Dave W0DCX


Mike Malone
 

To all of the participants in this thread, first... thank you!  Great information.  Now second, can I print this in the QRP Quarterly?  I would need you all to give me an okay to do so.  I would edit it into an article and credit you all with it and probably present it as the thread it is.  I found it fascinating and informative, after all... we are all stuck at home and have wire and tuners.  This is timely for those with time on their hands.
Mike KD5KXF
QRP Arci Qrp Quarterly Editor


On Fri, Jun 19, 2020 at 10:20 AM Dave W0DCX <cwqrp73@...> wrote:
I want to thank everyone for their helpful and educational input.  I am going to study the resources provided and do some further experimentation with various random wire configurations.  .  If I discover anything of interest I will let you all know.  Best 73s!
Dave W0DCX


Dave W0DCX
 

Mike,
Okay from me.  I only wish I could have arrived at a definitive answer to my original question.  Even so, I learned a lot from the discussion and maybe others will also.  I plan to continue trying to understand and optimize my random wires in this fashion - it is fun and challenging.  The final result may be that there are too many variables and influences on measurements and therefore an empirical approach is best - i.e., just try stuff until something seems to work.
Dave W0DCX


J.K. Wright
 

A small 49:1 transformer solves a lot of.  Most any mismatched impedance divided by 49 can be easily tuned.


On Tue, Jun 23, 2020, 9:51 AM Dave W0DCX <cwqrp73@...> wrote:
Mike,
Okay from me.  I only wish I could have arrived at a definitive answer to my original question.  Even so, I learned a lot from the discussion and maybe others will also.  I plan to continue trying to understand and optimize my random wires in this fashion - it is fun and challenging.  The final result may be that there are too many variables and influences on measurements and therefore an empirical approach is best - i.e., just try stuff until something seems to work.
Dave W0DCX


John Nicholas
 

As a new Ham,  got my Tech and General Ticket Mar 14,  this has been one of the best things to read and re read that I have seen since I started studying for the exams in January.  I felt it impacted more then just tuning a random wire.

Thanks,  getting in an article form, even cleaned up email exchange would be great.

de KEoZUW John

On Jun 22, 2020, at 3:35 PM, Mike Malone <kd5kxf@...> wrote:

To all of the participants in this thread, first... thank you!  Great information.  Now second, can I print this in the QRP Quarterly?  I would need you all to give me an okay to do so.  I would edit it into an article and credit you all with it and probably present it as the thread it is.  I found it fascinating and informative, after all... we are all stuck at home and have wire and tuners.  This is timely for those with time on their hands.


Mike Malone
 

John, I agree.  I need to hear from Jim and the others in this thread that haven't emailed me first.  Jim N5IB has been a guiding influence on antennas for me, ever since I heard a presentation he gave.  Good stuff gentlemen.

Mike KD5KXF


On Tue, Jun 23, 2020 at 11:17 AM John Nicholas <stnick@...> wrote:
As a new Ham,  got my Tech and General Ticket Mar 14,  this has been one of the best things to read and re read that I have seen since I started studying for the exams in January.  I felt it impacted more then just tuning a random wire.

Thanks,  getting in an article form, even cleaned up email exchange would be great.

de KEoZUW John

On Jun 22, 2020, at 3:35 PM, Mike Malone <kd5kxf@...> wrote:

To all of the participants in this thread, first... thank you!  Great information.  Now second, can I print this in the QRP Quarterly?  I would need you all to give me an okay to do so.  I would edit it into an article and credit you all with it and probably present it as the thread it is.  I found it fascinating and informative, after all... we are all stuck at home and have wire and tuners.  This is timely for those with time on their hands.