Antenna Tuners

[W4RFM]

So as not to hijack the tanks and tuners thread, let me ask this:
Why does the antenna tuner capacitors, need to be able to handle more current than the loading capacitor of the transmitter connected to it? That is, if we have a 200 watt transmitter with "receiver" spacing on the loading cap, why would we need 1/8" gap capacitors in the tuner?

[K1JJ]

 Hi Bob,

The voltage requirements (plate spacing) depends on where the cap is located in the tuner circuit and what the impedance is at that point.

For example, when using a 50 ohm link tuner input with a variable series cap, the impedance is usually 50 ohms (low impedance)  in that input circuit. This requires a lot of capacitance but the voltage is low and the current higher. This cap will usually remain low impedance no matter what the openwire presents to the output tank. At 50 ohms, this cap would handle about the same current and voltage as your typical 50 ohm pi-network loading cap in the transmitter. (at a given power level)

The bigger second cap or caps in the tuner are associated with the tank coil and this means higher voltage at times and wider plate spacing requirements. The voltage will be proportionally higher and the current lower. However, there will be bands where the impedance will be low and the voltage will be low with higher current. It will vary greatly across the tank capacitor(s) depending on the feeder impedance for a given freq.


So, if we happen to get a VERY low impedance, say feeding a 75M dipole on 160M, the circulating current in the tank coil and capacitor can be HUGE, with small voltage. This may require a series feed arrangement. Either way, low impedance output feeds are special animals that require heavy components in the tuner output tank components, antenna and feeder wires to reduce power losses.  Maybe this is what you were wondering about. Low impedance feeds could cause tuner tank currents to be larger than the typical current in a 50 ohm pi-net capacitor. (The input circuit will remain roughly about the same at 50 ohms for sake of discussion) You may see the same low impedance high current situation on the higher bands if you happen to have a specific feedline length that creates a LOW impedance feed situation.

Hope this helps.

T

[Mike/W8BAC]

Just off the top of your head Tom? Nice explanation! You helped me understand this as well. Thanks.

Mike

[K5UJ]

I think it's safe to say that if you need giant components to handle current because the Z being matched is extremely low, then you need to change something because the tuner is acting like a band aid.  at typical ham power high currents mean loss somewhere, either heat, coupling to ground or both.

[K1JJ]

Thanks, Mike.  I answer the easy ones and get the gurus to come in when the going gets rough...

Rob:

Good point about heat.  Yes, heat tells us something needs a configuration change or to make the the tuner larger or the antenna more suited to the job at hand.

In the case of a tuner, we can go to series feed rather than parallel feed if the impedance is very low. This may help a lot.    

As we know, the usual reason creating low impedance feed is employing an antenna for multiband use.  A typical example is a 75M openwire fed dipole used on 160 - OR  used on a higher band where the input works out to be of low impedance.  This can be solved with separate antennas, of course, but depends on our available space. (or changing the feedline length to favor a particular band(s).)

T

[KA2DZT]

Just off the top of your head Tom? Nice explanation! You helped me understand this as well. Thanks.

Mike

Mike

That's just FB for you.  But, as usual, Tom has me totally confused.

Fred

[K5UJ]

Tom Thanks; what I had in mind were dipoles on 160 too low to the ground (coupling to ground) for example, or an inverted L with the vertical part going up only a short distance like 20 feet (Z under 10 ohms most likely).  Both will be low Z antennas and cases where it would be better to fix them rather than try to match them.  Fred, I am normally very confused, just ask Rodger who patiently explains things to me.  This time I don't even know enough to be confused.

[W4RFM]

Thanks for the info everyone, I am building a 300 watt AM rig for 80 & 40.  I planned an antenna tuner on the McCoy Ultimate Transmatch design, and was wondering how close of spacing on the 200/250 pf variable caps I could get by with.  I have a nice Johnson roller inductor for the "middle" of the circuit.

[K1JJ]

Bob,

So you'll be running around 1500 w pep output or so...

Suggesting a capacitor plate spacing is a difficult thing. The gap will depend on humidity, impedance of the circuit and feedline, rouge audio peaks, dust, frequency, transcients in the rig, etc.

Many of us just choose to use a vacuum variable cap and forget about it.   That said, I would take a look at the spacing in a Johnson KW Matchbox and see what that is... maybe 3/16" - 1/4" or so?  I'm sure the guys who have used air gap caps for big tuners can suggest a good spacing value.  

In the past when using air dielectric caps, many times I would install a cap and later have to replace it for a bigger one due to arcing.  It pays to put in as big as you can from the start for room to grow and be able to handle the worst-case feedline matching situation, etc.

T

[WA1GFZ]

I built the McCoy tuner once and found it a HOS only good for low power. Any tuner with a balun on the high impedance output is pretty much junk unless you use giant components. It is a hi pass network so does not attenuate harmonics

[WD8KDG]

Thanks for the info everyone, I am building a 300 watt AM rig for 80 & 40.  I planned an antenna tuner on the McCoy Ultimate Transmatch design, and was wondering how close of spacing on the 200/250 pf variable caps I could get by with.  I have a nice Johnson roller inductor for the "middle" of the circuit.

Somewhere around 1977 I built McCoy's Ultimate Transmatch using the plans in the 1976 ARRL handbook. Built it with components listed, three T-200-2 cores, etc. It will handle 350 watts from a Junkston "500".

Trick is having enough room for a proper antenna, tunna then isn't being asked to do the impossible. As far as other designs, K1JJ, no doubt they work. But heat is heat, power is power, SWR is SWR; the result of the mix with your antenna is only spread over a larger area and components are not stressed as much. Any tunna is a compromise, just some are more efficent.

Craig,

[W3GMS]

Thanks for the info everyone, I am building a 300 watt AM rig for 80 & 40.  I planned an antenna tuner on the McCoy Ultimate Transmatch design, and was wondering how close of spacing on the 200/250 pf variable caps I could get by with.  I have a nice Johnson roller inductor for the "middle" of the circuit.

Hi Bob,
I am curious to what you will be using for an antenna.  If you said that before, I missed it. 

For coax fed antenna's the pi network in your transmitter should be up to handle the task for a resonant dipole. 

For open wire line you will need a tuner.  Since your in the planning stages, I would like to suggest you dump the balun approach and go to a link coupled parallel resonant type tuner.  They are simple and have very low loss.  You only need one variable cap with wide spacing to handle the voltage across the coil.  Since it's parallel resonant, the voltage will be very high.   Tom, K1JJ has written the one he built up and I know of lots of folks that duplicated it with great success.   

If you just using a random length wire a simple L matching tuner is all you need.

Have fun!

Joe, W3GMS

[K5UJ]

I think that McCoy tuner was the basis for the Murch tuners. 

The downside to vac caps is you have to deal with mounting them and turns counters.  It can take some time cranking them to find the right value for a match.   but after the first time you jot down the numbers.

Air variables don't leak but the trick with them is finding the balance between range and v. breakdown.  Big bread slicer may only have 200 pF range, unless you find one (almost unobtainium) that's two feet long or more. 

Here's your low loss low Z feedpoint coil hi hi:

http://www.ebay.com/itm/GIANT-JOHNSON-ROLLER-RADIO-TRANSMITTER-RF-COIL-50KW-/150660537719

[ke7trp]

Mine has a 35K vac variable on output side.  I also have fixed Broadcast type mica caps that I can plug in on the 50 ohm side for more range if needed. There is less chance of arcing and more transfer if the plates are full meshed. Its when they are open or 1/4 way meshed that you tend to have issues with heat or arcing. 

I really dislike air caps.  One mistake or arc over and then they have a tendancy to keep arcing. You end up having to clean them if this happens and it will...  Branch into antenna, Antenna on ground, Wrong band ect..   



C

[WA1GFZ]

yes but air caps don't leak

[Jim, W5JO]

And they can arc.

[WA1GFZ]

My Fugly has many an arc mark on the 1/4 inch spaced cardwell caps. Still works great at 29 years old.

[Opcom]

If the power is known and the impedance is known, can the circulating current be calculated?

[W4RFM]

Nice input, but all of you are missing the point of my original question.  The output capacitor (LOADING) on three different Handbook designs for a 300/500 watt amplifier / transmitter has receiver spacing.  Yet you are telling me I need a vacuum variable or something rugged to start off the match! Please explain where all that energy came from between the PL259 on the TX and the PL259 on the Antenna Tuner.  I am very confused.  If I can load my amp with 1/16th spacing on the loading cap, why can't I match it to the antenna with the same type product? The antenna tuner is matching impedance not creating more voltage/current! Yes, No?

For what it's worth antennas will be a north/south, and a east west (separate antennas) MFJ Doublet with 4:1 balun.

[KA2DZT]

Broadcast type caps work for the loading cap because you using a low impedance antenna connected to the pi network.  Run your xmtr with no antenna connected (hi impedance load or no load) and you will see the broadcast cap arc.

In an antenna tuner there may be high impedance loads that you are trying to match to your 50 ohm output of the xmtr.  These high impedance loads will result in higher voltages appearing across the tuner's capacitor.  Hence you need tuning caps with much wider spacing.

You haven't created any new energy, it is just that at the point the tuner is along the transmission line there may be less current but more voltage.  Still the same power as you had at the output of the xmtr at 50 ohms where you had greater currents but less voltage.  P=IV or Isq x R or Vsq/R.  Matching into high impedance balanced lines, you will have much higher voltages for the same amount of power  (P=Vsq/R).  So, if the impedance is high, the voltages across that impedance will be much higher than the voltage across a 50 ohm load for the same power level.

So, it depends on what antenna you're using, what transmission line you're using and what type of tuner you're using.  So, in most cases, caps used in antenna tuners need to have wide spacing, to handle whatever high voltages may be present at any given event.

Hope some of this helps and I'm sure if I got something wrong there will be more follow-up posts to further confuse the topic.

Fred

[ke7trp]

I am sorry for not answering that as well. The reason is as stated above. Its 50 ohms. There is not much voltage there at 50 ohms.

C

[Opcom]

I don't know why a vac. var. is wanted on the input to a tuner that is being driven by a ham transmitter with a 50 Ohm output and uses a BC variable for loading, except that something may to arc if the load is wrong. Pick one, the amplifier's loading cap or the tuner's input cap.

I could add to what Fred said, say the power is 1000W at the 50 Ohm output of the amp. That is 223V RMS, 316V peak. So the BC cap is OK even for some legal limit amps. "OK", but it is still small-spaced and a compromise for cost/chassis space sake otherwise they'd have put in a heavy duty one with 2x the spacing, and it would be 4x the size too.

The input side of the tuner if all is perfect should also be 50 Ohms, and that same low voltage, but if things are not perfect and if the cable between the two is longer than a few feet (as indicated above) then the fraction of the wavelength of cable there changes the voltage and current relationship. Because the tuner does not know what it is getting, the tuners usually spec a much higher voltage cap.

That is explained somewhat in the handbooks although they don't go into a huge amount of detail, making it seem less important. (an example of this in common practice is where CBers, who usually know nothing about it, always are advised to use the shortest possible cable between the radio and the footwarmer)

Some tuner designs also can accept a wider range of impedance on the input and so have a higher voltage capacitor. 1KW in 200 Ohms, is 630V peak, and 200 Ohms is not that uncommon if something isn't tuned just right.

The parallel to this, for an amplifier, is that if the amp has a roller inductor instead of a tapped bandswitch coil, and the roller is not ganged with the tuning cap, it can be made to match the tube to a much wider range of output impedance and could need a wider spacing on the loading cap.

The amps in the handbooks with the BC RX caps are probably for 50-75 Ohms, not intended for higher impedance.

This whole thing works the opposite way as well. As the impedance is lowered, the current is higher for the same wattage, which is the question in the post - current was mentioned.
1KW to 50 Ohms =223V@4.5A
1KW to 25 Ohms =158V@6.3A

and for those KW (carrier) BC rig folks with no 'low side' switch.. the peak current is more challenging at 9A. Oh that's not much except it all has to flow on the skin, as the skin effect.

If somewhere in the tuner, the Z is very low like 10 Ohms, then it is 20A. At some point things will overheat and even the feed lines to the antenna can become burned if that 10 Ohms is at the output of the tuner.
On the other hand if a balanced feed line or a wire shows a 2000 Ohm load to the tuner, then it's a voltage issue like 4KV peak. RF likes to arc sooner than DC, so an even wider spacing is needed.

Basically the same as the previous answer with a couple different examples. I'm no expert on it though, only seen or measured individual setups. Hope my math and reasoning was right.

[KA2DZT]

Good post Patrick,

I checked your math, it looks correct.

There are many factors to consider.  But just looking at a link couple tuner,  you really don't need a cap on the link to ground.  The pi network should be able to handle most low to few hundred Z loads reflected back into the pi network from the tuner.

Using balance lines to feed antennas, high impedances can appear at the tuner.  Depends on what is being done.  Example, trying to feed a 40M signal into a 80M dipole.  End result is you're feeding the signal at the high impedance point (voltage feed).  Now depending on the length of the transmission line between the dipole and the tuner, the tuner can see this same high impedance to anything lower.  If the balance line is a 1/2 wavelength or multiple, the same set of conditions that appear at the dipole will appear at the tuner.  This worse case may be too much for some tuners to handle.  Changing the length of the balanced line should improve these conditions.

A lot depends on what the operator is attempting to do.  Trying to use one antenna for all bands can present any number of problems for a antenna tuner to deal with.

Having said all of this,  I use resonant antennas for each band.  No tuners, no baluns, KISS.

Fred

[flintstone mop]

Just off the top of your head Tom? Nice explanation! You helped me understand this as well. Thanks.

Mike

Mike

That's just FB for you.  But, as usual, Tom has me totally confused.

Fred

151 Watt Fred........This is "the other Fred"
The info floating around here is mind boggling at times. But I'll take what Toms says and do a little more research to dig deeper. Understanding antenna principles and the various ways that they will accept RF and radiate it, is complex. Most folks in Amateur radio are constantly trying to get a free ride to have an efficient antenna at these low freqs (160M and 80M) and forget the physics involved. We gotta do it by the book. It's good to try the challenge of understanding RF principles and the science behind cuz there ain't many of us left who are RF engineers.......For the really new folks it is all cook book building and not knowing how or why. Analyzer? waz that?? Reactance???
There is another thread that an OP wants to actually know the voltage and current points on an open feeder line so that he knows precisely how much capacitance or inductance to insert into his system.
I like it simple......build a good design balanced tuner and have about 200 feet of wire out there, 6 inch spaced ladder line, about 50 feet high and it's like magic.........

fred

[The Slab Bacon]

Just FWIW.................

I run a difficult arrangement: A lot of power into a short antenna. (I have no other choice, its cheaper than buying another home....) And I use your basic "T" type tuna. (I know, Frank, its a high pass filta) I use it because I'm lazy and it is able to match up a lot of really ugly complex impedances.

I had to rebuild it 3 times due to arcage and sparkage.  After inventing 7 new cuss words, I decided to run it with the cover off and see exactly where it was unhappy. Low and behold, I saw the revelation! ! ! On some bands the inpoot cap would flash over, and on others, the outpoot cap would flash. I finally ended up with 1/4" spaced bread slicers to cure the problem.

One must keep in mind that a tuna is an impedance matching network, a variable transformer, if you will. And it starts at the outpoot side of the inpoot cap. The "T" junction (junction of both capz and the hot end of the inductor) can carry some real obscenely high RF voltage potentials depending on what you are trying to match to the transmitter. Henceforth, the need for wide spaced breadslicers and/or vacuuuuuum caps.

We have discussed this whole subject many, many times ad nauseum. Sometimes it is just better to accept the fact that that is the way it is, rather than trying to re-invent the wheel. Just accept the fact that you will need robust caps for a good tuna, build it and move on to the next project. Most of us that have been playing that game for a while have already been through this and learned the hard way.
We give out advice to save others from falling into the same holes that we did.......

Just my $.02 worth