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Author Topic: Doesn't vacuum variable flange resistance hurt magnetic loop performance?  (Read 3851 times)
VE7TIT
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Posts: 62




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« on: October 20, 2012, 08:36:39 PM »

It looks like vacuum variable capacitors don't have solderable connections but are mounted with flanges. Doesn't that contact resistance hurt the performance of a magnetic loop? Isn't the contact resistance of the flanges pretty much the same thing as the wiper resistance with a typical air variable? Thanks for any thoughts you may have on this. I know nothing about air variables except what I see in the photos on eBay.

(The reason I'm considering a vacuum variable is that the performance of my one-meter-square loop is astonishingly good! These things really work!)  Grin
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VE3WMB
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Posts: 289




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« Reply #1 on: October 21, 2012, 07:46:32 AM »

The simple answer to your question is yes this mechanical connection can be a source
of loss in a Mag Loop. However, unlike the wiper connection on an air variable capacitor, the mounting
flange for a vacuum variable doesn't have to move.  Just yesterday I was reading about a loop constructed by Frank N4SPP
( a very detailed web page BTW :
http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm).

I noticed that towards the bottom of the page that he used stainless steel hose clamps to make the connections to his vacuum variable, but his later measurements showed that this solution was introducing unacceptable losses.

Often these sorts of losses are often not such a big issue on the higher
bands (i.e. 10m) because the radiation resistance of the typical 3 foot Mag loop tends be higher there.
Where it really kills performance is when you move down in frequency (i.e. 40m) because the typical
3 foot loop is very small for that band. What tends to happen as you move down in frequency to a lower band is that the mechanical losses become proportionally very large as compared to the radiation resistance of the loop. As the radiation resistance gets smaller this causes the efficiency to go down the toilet. I think that practically, if you can size the loop appropriately for the bands of interest, you can tolerate some mechanical losses and still have a very usable loop.


BTW if you are not already a member, why not join us on the MagLoop Yahoo group. You will find a lot
of like-mind Hams interested in both commercial and home-brew Mag Loops.

Cheers

Michael VE3WMB

It looks like vacuum variable capacitors don't have solderable connections but are mounted with flanges. Doesn't that contact resistance hurt the performance of a magnetic loop? Isn't the contact resistance of the flanges pretty much the same thing as the wiper resistance with a typical air variable? Thanks for any thoughts you may have on this. I know nothing about air variables except what I see in the photos on eBay.

(The reason I'm considering a vacuum variable is that the performance of my one-meter-square loop is astonishingly good! These things really work!)  Grin
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VE7TIT
Member

Posts: 62




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« Reply #2 on: October 21, 2012, 10:10:50 AM »

I know nothing about air variables except what I see in the photos on eBay.

I meant to say that I know nothing about *vacuum* variables, of course.  Embarrassed
« Last Edit: October 21, 2012, 10:15:59 AM by VE7TIT » Logged
VE7TIT
Member

Posts: 62




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« Reply #3 on: October 21, 2012, 10:15:39 AM »

The simple answer to your question is yes this mechanical connection can be a source of loss in a Mag Loop...

BTW if you are not already a member, why not join us on the MagLoop Yahoo group. You will find a lot of like-mind Hams interested in both commercial and home-brew Mag Loops.

Thanks. That's ironic that the (very) expensive, high-tech solution may actually be inferior to a simple sliding copper plate arrangement at the low end of the tuning range. (Probably higher Q because of smaller size, though?)

I am a member to the Yahoo magloop group, but joined so long ago (years...) that I had forgotten. Thanks for the reminder. I'll drop by again.
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N3OX
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« Reply #4 on: October 21, 2012, 09:48:56 PM »

It looks like vacuum variable capacitors don't have solderable connections but are mounted with flanges. Doesn't that contact resistance hurt the performance of a magnetic loop? Isn't the contact resistance of the flanges pretty much the same thing as the wiper resistance with a typical air variable?

Nah, not the same thing.  The smaller flange on the vacuum variable I have is at least 10 cm^2 of silver plated contact.  Of course the current isn't going to get past the edges if the clamp is fitting properly but it's still a big contact area clamped tight and left alone.

In addition, I believe the equivalent series resistance is specified with the flange connections included.   I'm not 100% sure of that but I think it's so.  These capacitors are designed to be used with 40A or 70A or 90A of RF current flowing continuously, and as far as I know the flange connections are part of that high-current-handling design.

I'm pretty confident there's no better capacitor for a high power wide tuning range magloop than an expensive vacuum variable if the cost is not a major consideration.
« Last Edit: October 21, 2012, 09:51:38 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
VE7TIT
Member

Posts: 62




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« Reply #5 on: October 23, 2012, 08:33:32 PM »

I'm pretty confident there's no better capacitor for a high power wide tuning range magloop than an expensive vacuum variable if the cost is not a major consideration.

I think you're right. I didn't realize that was silver I was seeing. Thanks!
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K5RPM
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Posts: 15




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« Reply #6 on: March 22, 2013, 10:11:53 PM »

Gents,
I built a magloop with copper foil, .008" thick, and utilize a surplus Russian vac variable cap.  I have the foil clamped very tightly to the cap ends with the supplied cap mounts/straps.  Would it be beneficial to use Penetrox on the connections, or leave them dry and just check tightness semiannually?
Thanks,
Ron
K5RPM
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N4CR
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Posts: 1673




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« Reply #7 on: March 23, 2013, 08:09:37 AM »

I have mixed feeling about putting Penetrox there. It's no where near as conductive as direct contact, but direct contact will be compromised by the weather.

I would say you would know this answer by how the antenna performs after you add Penetrox. If it didn't change in operation, or if it acceptable then it should be more stable over time.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
G7DMQ
Member

Posts: 40




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« Reply #8 on: March 23, 2013, 10:10:19 AM »

I have a feeling this might get me flamed -as it's not a popular point of view!

I've built a few loops and I'm less and less convinced how much difference the loop resistance actually makes to the 'efficiency' (see later). 

I know every site you look at says it has to be as low as possible - and it's probably a good idea to make it as low as you can, but although it's subjective - I've not seen much difference between a loop of thin wire compared to a 2" wide x 1/16" thick copper strip TIG welded to the capacitor - or copper pipe sections welded together.

The biggest noticeable difference is, as the resistance increases, so does the tuned bandwidth - so I wonder if the 'efficiency' is referring to the power density more than the loss?

The copper strip loop shows a bandwidth of about 4khz where as the wire is about 20khz - but it doesn't seem to make a huge difference to what I can hear or who can hear me!

I posted this observation somewhere else - and got told I was an idiot!  That may be true - but I'm only going on my observations.  Other peoples may well be different!

I have had issues with the wiper on a capacitor - cured by using a butterfly.  I wonder if the oxide on the shaft/wiper is acting as a semiconductor or something?

At the moment I'm playing with a binary switched capacitor instead of a variable.  Using 9 high voltage capacitors from 1pf up to 500pf connected via relays in combinations to give me 1pf increments from 0 to 1000pf.  Controlled by a microcontroller - it will give very fast and accurate tuning.  You might think that the resistance of all the relay contacts would kill it - but it seems not!  Sadly the first version let the smoke out as the voltage was somewhat higher than the rating of the relays or capacitors!

Si
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N3OX
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« Reply #9 on: March 23, 2013, 10:47:37 AM »

I know every site you look at says it has to be as low as possible - and it's probably a good idea to make it as low as you can, but although it's subjective - I've not seen much difference between a loop of thin wire compared to a 2" wide x 1/16" thick copper strip TIG welded to the capacitor - or copper pipe sections welded together.

You should be a little careful. Unless you do side-by-side comparisions (with automatic detuning of the "unused" loop at the time!!!) and take lots of good statistics, you'd have a hard time seeing a loss like 3dB or 6dB... maybe even more. Furthermore, coupling to nearby objects could be large and change the equation in your specific situation in a way it wouldn't in someone else's.

In a lot of situations an amount of loss that you don't notice subjectively does not, in fact, matter on the ham bands. A lot of contacts we make have many tens of dB surplus signal margin! But in a weak-signal situation it MIGHT matter

I'm sorry you got called an idiot. That's not fair.

But sometimes we need to have a pretty sharp separation between objective measurements and subjective results because an excessive reliance on subjective results hurts peoples' understanding. Wire loops and fat conductor loops really don't work the same; if you find that they work basically the same on the air that's really just saying something about how many dB actually matter in your radio habits.

Now, we also have a problem with people who can't accept that "good enough is good enough" and even more of a problem who treat efficiency overkill as some sort of proxy for their technical prowess. You wanna trade off a few dB for convenience, go for it, I don't mind. Ham technical advice often tends toward platitudes and "rules of thumb" like "NO MECHANICAL CONNECTIONS!!!" That is also harmful to understanding. But we SHOULD have an honest and objective conversation about which things matter and how much.

We shouldn't base the choices we make in antenna design around platitudes about "best practices" nor should we base antenna design ADVICE on on-air results, but we do both of those things a lot in this hobby.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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Posts: 12913




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« Reply #10 on: March 23, 2013, 11:23:50 AM »

Wider bandwidth on a physically small antenna is a sure indication that you have increased resistive losses. Increased resistive losses means more of your signal is converted to heat thus less is actually radiated. As N3OX said though, you may not see a performance difference jump out at you unless you really do careful A/B comparisons with all other variables held as constant as possible.
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G7DMQ
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« Reply #11 on: March 23, 2013, 11:44:24 AM »

As I said, my observations are subjective and not made under laboratory conditions - but most Hams do not operate under lab conditions.  Many people are delighted with random wire antennas after all!

I've tried to be as subjective as I can - and I do aim to make good antennas.  I'm just not convinced, from an amateur point of view, it's worth going to the ends of the earth chasing every last micro ohm.

Also, at the capacitor, the voltage is at a maxima and the current a minima.  Therefore, resistive heating losses at the capacitor I^2 R will also be a minima compared to anywhere else in the loop.  That is, unless you believe the current and voltage are constant around the loop - as some people do!  So, the connection to the capacitor is the least important.

I'm sure for commercial / military applications where cost is less of an object, it is worth the effort - but for most hams, the wider bandwidth is actually a benefit which MAY outweigh the lower efficiency.

The Excell spreadsheet loop calculator thinks my wire loop should be about 1% efficient!  The copper strip should be about 60% IIRC.  If they are accurate - it is a difference that should be very obvious?

I don't want to upset anyone - I just like experimenting!

Si
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N3OX
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« Reply #12 on: March 23, 2013, 12:38:00 PM »

I've tried to be as subjective as I can - and I do aim to make good antennas.  I'm just not convinced, from an amateur point of view, it's worth going to the ends of the earth chasing every last micro ohm.

Depends on what you're trying to do. If you're trying to compete with a full size 40m vertical for DXing, maybe you care about small resistances.

Quote
Also, at the capacitor, the voltage is at a maxima and the current a minima.  Therefore, resistive heating losses at the capacitor I^2 R will also be a minima compared to anywhere else in the loop.  That is, unless you believe the current and voltage are constant around the loop - as some people do! 

The uniformity of the current depends on how big the loop is. There will be quite a lot of current through the capacitor for small loops.

Quote
I'm sure for commercial / military applications where cost is less of an object, it is worth the effort - but for most hams, the wider bandwidth is actually a benefit which MAY outweigh the lower efficiency.

Maybe, but hams need to have an "informed consent" about these things.

Quote
The Excell spreadsheet loop calculator thinks my wire loop should be about 1% efficient!  The copper strip should be about 60% IIRC.  If they are accurate - it is a difference that should be very obvious?

If those numbers are accurate there should be around 17dB difference which is getting into the range that should always be pretty noticeable "from memory"

But if there's something else in the environment like earth losses, that might dominate the total loss resistance. The other thing is that the Excel spreadsheet calculator doesn't take into account the fact that a loop over ground (if it's vertical and fairly low) actually has higher radiation resistance than a loop in free space. Over perfect ground the radiation resistance doubles, which could give a couple more dB.

Depending on how you're doing the connection to the loop, coax shield currents could improve the performance of a lossy loop. There's all kinds of stuff that can happen.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
G7DMQ
Member

Posts: 40




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« Reply #13 on: March 23, 2013, 01:43:15 PM »

Dan,
I am trying to compete with a half wave horizontal for DX - and on the whole the loops are about 1 to 3 db down on the dipole - though often Lower noise which makes up for the lower signal.

I have a half wave on 40m vertical - but it's not good in this QTH.

I wish I could post photos from my iPad - I've just made a deliberately high resistance loop out of steel fencing wire and choc block connectors!  Fed via coax with a gamma match, also made using connector blocks.

My analyser shows a bandwidth close to 200khz on 40m and it's about 1db down on the dipole.  I expected it to be terrible - but it's actually pretty good!  Just talked to a station in Ufa in Russia, just this side of the ural mountains - must be 1800 miles.  Both of us on 100w.  Best of all - no tuning!  SWR is 1.6 at 7.000 1.0 at 7.100 and 1.3 at 7.200.

Don't believe me?  Try it!  1/8" Galvanized fencing wire.  1.2m diameter and a 500pF variable only rated at 1kv but it doesn't arc at 100w.

I have an FG-01 analyser which is where my SWR and bandwidth figures have come from.

Si

Si

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G7DMQ
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Posts: 40




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« Reply #14 on: March 23, 2013, 04:35:47 PM »

Quick video of fencing wire loop
http://www.youtube.com/watch?v=bphwCvAvSh4&sns=em
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