The AM Forum

I have been devouring the 1926 edition of the Handbook and among many other quriosities, I note the complete lack of the expression SWR.  Power meter is not part of the vocabulary and power is not even talked about much.

Other than light bulbs, the only instrument ever discussed in this regard is the antenna ammeter.  After reading how it is used and why, it seems like a very useful instrument although I have never seen or heard anyone discuss it's use.

I found a few do-it-yourself projects on the web and then thumbed the MFJ catalog and find that they have all sorts of them including a dual version for balanced feed line and clamp on types.

Before I jump and buy one of these, I am curious why I never hear of anyone using them.

I also note that MFJ rates them in amps (surprise) and the max is 3A.  Assuming 50 ohms, that would be 450W max.  Is this the right math or am I missing something.  What if the load is not 50 ohms and/or totally unknown?

I guess I am trolling for thoughts on the use of these instruments today with all the inexpensive SWR gear out there.

js

I have used them in my open wire feeders since 1986. I wouldnt think of using any type of ant feedline combo without one except regular coax resonant.  I buy antique ones at festers  for a buck and restore them. the one I have now is a G.E.C.O. last patent date on it of 1919.  Paid 50 cents for it at Barryville.  Prolly saw spark signals.

make sure they say "internal thermocouple" on the meter face - or that it has the matching external thermocouple block with the matching handwritten serial numbers on both the meter face and the thermocouple block. They are calibrated together as a unit. Quite useless apart from each other.

The big  "fan" shaped Weston gray colored ones are my favorite.

A real thermocouple ammeter responds only to the current drawn by the antenna radiation resistance - it ignores the inductive or capacitive reactance.   Unlike wattmeters it will continue to tell the truth even on an unmatched (high SWR or high reflected power) antenna system.

You are correct, 3 amps indicated on a thermocouple ammeter in series between a transmitter and a 50 ohm resistive antenna system.

P = (I squared) * R
  = 9*50 = 450 watts.

Amateur use of these meters is usually just "tune transmitter for maximum RF current".  Commercial use is as a precision power indicating device with carefully measured known antenna or load R.

I don't know if the MFJ products are thermocouple ammeters.  Let's go check...  I can't tell but the documentation seems to be saying they only pay attention to antenna resistance like a thermocouple meter.

Interesting side note - you can calibrate an unknown thermocouple ammeter using regular 60 Hz AC power.  Put a known-calibrated and unknown meter(s) in series with each other, a Variac or other source of variable AC, and a load like a light bulb - turn up the juice and get a reading on the known-calibrated meter well up the scale.  Adjust the unknown meter to match the calibrated one.  The actual  current doesn't matter that much - you are just bringing the one meter into line with the other.

Steve WD8DAS

Here's a "trick" I use to scale (properly) a thermocouple r.f. ammeter:

Suppose (like I do) you have an r.f. ammeter that has a full scale reading of 15 r.f. amperes (11,250 watts into 50 ohms !)... and you want to convert it to an r.f. ammeter that has a full scale reading of 5 r.f. amperes (1250 watts into 50 ohms). More specifically, when the actual antenna current is 5 r.f. amperes, the r.f. ammeter will read full scale ("15 r.f. amperes").

Do the following:

Make up a 3:1 r.f. transformer of the same type that is used to build a typical Class E transmitter.

For example: a 3 turn primary and a 1 turn secondary, with four (4) SB-1020-43 cores... if you are going to use it with 375 watts of AM (at carrier) on 75 meters.

[The lower the frequency of operation, and the higher the current passing through the primary, the more cores you need to prevent saturation and associated heating of the cores. Since there is no DC, you can use fewer cores if you use something like a 12 turn primary and a 4 turn secondary.]

Place the r.f. ammeter across the secondary (the 1 turn side). Run the r.f. current you wish to measure through the primary (the 3 turn side).

That's it. You're all set.

The current in the 3 turn primary must equal 1/3 of the current in the 1 turn secondary. If the primary current is 5 r.f amps... then the secondary (through the meter) current will be 15 r.f. amps. Meanwhile, the r.f. ammeter's sensor looks like (essentially) a short circuit across the secondary of the transformer... so there is (essentially) no voltage drop associated with the antenna current flowing through the primary.


I've done the same thing to convert a 500mA r.f. ammeter to read 5 amps full scale. It also worked perfectly.

I used a 1:10 transformer (1 turn on the primary and 10 turns on the secondary).

When the antenna current is 5 amps, the current through the meter is 500 mA.

A thermocouple r.f. ammeter responds to the r.f. current passing through it. It is calibrated under the assumption that the r.f. current is a sine wave (no modulation).

If the SWR on the line being measured is 1:1, then the r.f. current will be the same anywhere along the line. If the SWR on the line being measured is 3:1. Then r.f the current at one location can be 3x as large as the r.f current in another location. Therefore, it is simplest to use the thermocouple based r.f. ammeter at a location where the SWR is 1:1 (e.g., between the transmitter and an adjusted tuner).

The reading on the r.f. ammeter does not depend on the impedance of the cable it is used in conjunction with... only the r.f. current flowing through the conductor that passes through it.


Best regards
Stu

 a ammeter is t it's best when you hve a tuner that can present a "good" swr over several different settings to a swr bridge. using a ammeter in the output and combining that maximum current reading with the "good" swr on the bridge will give you he real best setting for maximum power transfer, which is what you  are actually after.

50 ohms is one coaxial standard, not necessarily the actual impedance. If you look at at the 75 ohm dipole standard (RG-11) then your talking  675 watts, "magically" close to the output of a KW back in the day.  Of course, there's ladder line with higher impedances.

We seem to have been fixated on 50 ohms so long and for solid state work that we tend to forget that most legacy rigs had variable impedance output.
Some were advertised from 35 to 600 ohms. ! 
     

I guess the one important usage of a RF ammeter from way back was using it to make sure you had equal current on each conductor of your transmission line. Equal current demonstrated proper energy transfer to the load on a balanced aerial, such as a dipole. One of the accessories for my MK-214D is a MK-861, which is a pair of meters that plug right into a spot on top of the transmitter. (Man would I love to find that!!!!)

You can do that with wee light bulbs too. ;)

SWR is far too overrated. PO is where it's at. Tune for maximum smoke. If SWR is an issue, you won't need no steenkin swr meter to tell you. A good ammeter is a true ham's friend.

Mike

Assuming there is an extraneous current path that allows the currents of the two wires to be unequal, a good way to measure this is to place both wires through a current transformer, to measure the common mode current.

I made up a common mode sensor for coaxial cable applications (i.e., to see if there is any unbalanced current flowing through the outer part of the shield) as follows

I inserted a 2-foot  piece of RG8X through one of my zillions of SB-1020-43 cores (those cores and duct tape are my most often-used building materials), with a PL-259 connector at each end. But first  :), I wound 10 turns on the core. I terminated the 10-turn winding with a 10 ohm power resistor. I also placed one end of a suitable length of RG-174U across the resistor, and terminated the other end with an RCA plug.
 
To measure the common mode current in a coaxial cable, I insert the 2-foot piece of RG8X in series with the cable I want to measure.

If there is 100mA of common mode current flowing through the cable under test, then 10mA will flow through the secondary of my sensor's transformer... producing 100 mV across the 10 ohm resistor.

If the outer shield of the coax under test is not connected (and the missing shield current is flowing through some other path... forming a ground loop)... then one could have the following:

5 amps of unbalanced current in the coax => 500 Ma of current in the secondary of the 1:10 transformer => 5 volts across the 10 ohm resistor => 2.5 watts of heating of the 10 ohm resistor.

Yep. I love the fact that these things go begging to fetch a dollar 99% of the time.

They can also be very effective in ground system work in the lower ranges. Damn, I thought i was the only one that used em anymore.  ;D

In my beginning days on AM using a T-368 and a low flying (30feet high) dipole and wire strung everywhere on a small suburban lot. The antenna was fed by a true open wire ladder line and the 300 watts from the T-3 with 8A of current, made a lot of things in the tuner very hot. It was a constant battle of chasing the next component that would fail or melt or short out.
Thanks to this forum I have a very reliable and a reasonable signal on the air for 160M

Fred

Reading old radio magazines I often encounter the idea of increasing the antenna current as a way of increasing station range (getting out better). 

When adjusting the transmitter that notion makes good sense, but sometimes the issue being discussed is antenna adjustments - in which case you might just be changing the antenna resistance.  Lower radiation resistance would mean a higher current reading on the thermocouple ammeter - for the same power output. 

A higher reading could also mean higher efficiency due to a better-match to the transmitter, and better range - but one can't be sure without some other confirming measurement, like field strength or forward versus reflected power, or careful observation of final loading before and after.

Steve WD8DAS

Can someone provide an technical (theoretical analysis) reference for:


I'm not sure that I agree with the above... except to say that if you knew the current going into the combination of a resistive element and a reactive element (capacitive or inductive) in series, and if you knew the value of the resistance of the resistive element... you could calculate the power being absorbed (turned into heat or radiated) by the resistive element.

However, if you measure the current flowing through one conductor (assume that it has the same rms value as the current flowing through the other conductor) of a long length of balanced line (for example)... and if you make that measurement at a location that is more than a small fraction of a wavelength distant from the antenna, and if the SWR is not equal to one (almost certainly the case for most ham applications where balanced feed line is used)... then the current you measure will not be the same as the current flowing into the antenna itself.

This sounds non-intuitive... but it is less so if you consider that the total current on each conductor of the feedline consists of two currents: one flowing toward the antenna, and one flowing in the opposite direction. At some points, they add in phase, and at other points they subtract (out of phase by 180 degrees). This is, of course, where the expression: "standing wave ratio" comes from.

Stu

This thread has encouraged me to RE-install the ammeters, I have, on the inverted Vee system to see if I can optimize antenna current going into the Ether by playing with the taps on the K1JJ Tuna.

And one going out to the vert antenna project. But how do I remote PTT the transmitter to see any changes without looong 'dead band testing' and long walks to the antenna site and the shack??? About a 600 foot distance.
Something about a legal Ham freq and R/C device keeps screaming at me............more money to spend??
Fred

I have been pondering the same problem.  My solution is a wireless TV camera pointed at the meter and a monitor in the shack.

A PC wireless won't make it but there are many other inexpensive RF/TV options.

js

Be careful how you use an rf ammeter.  My Gates BC1-T is almost like a ricebox, with an output impedance range limited to about 50 to 70 ohms.  It wouldn't load into my 160m vertical over the entire band because near the band edges, the SWR is about 2.5:1, and the impedance that the transmitter sees when feeding that line is beyond its range.  So I put together a simple L-network tuner, with a switch to move the capacitive leg of the network either to the input or the output side.  The L-network can be adjusted to make the transmitter see 50 ohms @ 1:1 SWR anywhere in the band.

The Gates came with a 0-8 amp rf meter in the output.  I would adjust the network, and adjust the transmitter for the proper current through the built-in rf ammeter.  This worked very well until one day the transmitter wouldn't load up.  After a bit of  troubleshooting, I figured out that the rf ammeter had failed by blowing the thermocouple.  Thinking it was just a flaky meter, I stuck another one in line, and promptly blew it.  Turns out I had neglected to re-connect the feedline to the output of the L-network after a thunderstorm, and the network was looking into an open  circuit.  The circulating current between transmitter and L-network turns out to be tremendous with no load attached.  So I put in a 3rd rf ammeter, and tried to always make sure the load was attached before transmitting.  That worked FB-OM for a couple of months, until the meter blew again.  This time, the antenna changeover relay had failed, as the flexible copper braid leading to the moveable contact had come in two from metal fatigue.

So I just strapped a shorting wire across the terminals of the 3rd crapped out meter and repaired the  relay.  I use my old Mirage meter to null out reflected power, and then tune the transmitter for proper plate loading.  If I ever use another rf ammeter with that transmitter again, it will go between the L-network and the antenna feedline.

Luckily, I still have some good rf ammeters left.

I'll have to devise a way to clip the meter in circuit when it's experimenting time. It doesn't sound like a good thing to leave them in circuit continually for Ham use. Too many things happen.
I remember walking with a chief engineer friend taking current readings on a 5kw D.A. and he woud carry a flourescent bulb around for light and flip a switch at each 'dog house' to get the base current reading and then flip the switch again to take the meter out of circuit.
I'm still looking for a way to remotely turn on the TX from the antenna site so, I can see any changes in RF current.
I'm beginning to think that there will not be any dramatic changes that would make any difference on the receive side of the QSO. Prolly going from 5 radials to 30 would be a noticeable change. It would prove some of my thoughts of interpreting the readings from the MFJ 259 (Xc Lc RES. indications) with an ammeter and see how they coincide.

Fred

One final question before I move.

The MFJ dual ammeter for balanced line has a max range of 3A.

One more time, is this considered a full legal limit instrument?

How do I relate this to 50 ohm, 400 ohm and unknown loads.

js

Sir, you have just helped me figure out what the heck to do with the 0.5 and 0.1 amp thermocouples I have a handful of! Too PW for even the plastic radio, but now, a good solution. I could dig them out. NOS I think..  I am astounded I did not think of it, since I routinely use 60Hz current transformers.

Thanks,
Patrick

K9ACT wrote:

>The MFJ dual ammeter for balanced line has a max range of 3A.
>One more time, is this considered a full legal limit instrument?
>How do I relate this to 50 ohm, 400 ohm and unknown loads.

You sound frustrated - sorry for that.  Let's go over it again:  In amateur stations, where the antenna radiation resistance is not carefully measured, then the ammeter is more of a relative indicator.  You need to know the load resistance after the point you insert the meter to be able to figure out power.

The relationship of power and resistance is P = Isquared times R.  If the ammeter is the type that only responds to the power consumed by the antenna *resistance* you can calculate the power by knowing the resistance and the current.  For example, 3 amps into 50 ohms is 3squared times 50, or 9 times 50, or 450 watts.

A dual ammeter would be intended to insert into the two sides of a balanced line to ensure that the same current is flowing in each wire - that's the "balance" in a balanced line. 

If you have an antenna that used with an antenna tuner in the shack, that is, the antenna is not matched to the feedline, then the actual number of amps you measure in the shack is pretty meaningless.  But using it to check balance in the antenna system might be useful.

Steve WD8DAS

Fred wrote:

>I'm still looking for a way to remotely turn on the TX from the antenna site
>so, I can see any changes in RF current...
>Prolly going from 5 radials to 30 would be a noticeable change.

And it could result in the antenna current either going up or down, depending on how the antenna radiation resistance changes. 

Here's one scenario:  imagine if you had a ground-mounted vertical that had a radiation resistance of 50 ohms at resonance.  100 watts into the vertical would result in 1.41 amps on a thermocouple ammeter at the base of the antenna.

Some of that 50 ohms is ground loss resistance - say twenty ohms.  If you improve your ground system to eliminate ten ohms of the ground loss part, then the radiation resistance of the antenna will drop to 40 ohms, and you will read see 1.58 amps for the same 100 watts.

Interestingly, the SWR indicated at the shack would be a little worse, as the antenna no longer matches the 50 ohm line as well. 

Steve WD8DAS

I have actually heard hammy hambones over the air complain that something didn't  work right, because the SWR got worse after they added more radials to their verticals.

I'd bet that more than one bone-head has disconnected or taken up the extra radials, convinced that the antenna wasn't getting out as well.

Steve
You hit the nail pretty square on the head. I ( maybe WE) forgot that the z of a vertical IS 30 ohms!!!!!!!!
And Don, that's the rut we all get in all beause of these off shore radios and the 2:1 SWR. Nothing wrong with a little of the right kind of SWR...................hi
The magic might still be the RF ammeter, after all

Fred

Here is a .pdf (only 3 pages) of a US Patent dated November 23, 1937... which contains diagrams and a "specification" (descriptive information) that explain a lot about how thermocouple ammeters work.

Stu

Interesting discussion here with lots of food for thought.  However, I seem to learn more by doing than reading so....

I found this article on the web.... http://w5jgv.com/rfa-2/rfa-2.htm

I found a bag of 25, 3 amp Schottky diodes on Ebay for $5.

So, MFJ is going to have to wait for my $139 for that ready to go meter.

I have lots of meters to choose from but most of them do not specify the resistance and I am curious as to how one would go about measuring this.

My digital says 3.5 ohms and my needle multimeter pegs the meter.

It probably does not make much difference anyway. As I understand it, the absolute value is pretty meaningless and relative current is all we really need to know.

js

So how is this coupled to the transmission line?  If it's placed in series with one of the feeders won't the meter and associatted parts have to handle the full RF?  Or is this capacitively or Inductively coupled? 
 

All the thermocouple rf ammeters I have seen have the thermocouple mounted either on the inside of the back case of the meter, or the thermocouple unit is separate, with a couple of terminals to  go to the DC meter movement.  The whole thing is insulated, so the meter can be mounted on a metal panel.  I have never used one at a high voltage rf point, so I don't know how much voltage they could stand before arcing over.

I would assume that the W4JGV diode version would work the same way.  The internal resistance of the meter should be very low compared to the impedance of the circuit where the meter is inserted.  You can insert the meter in line, just as you would a hammy hambone wattmeter, or you can put it in series with one of the feeders of an open wire line to measure the current in the  feeders.  Ideally, two identical meters would be used, one in each feeder to monitor  line balance.  I have seen circuits use a double 4PDT knife switch to switch a single meter from one feeder to the other.  The extra poles are used to close the gap on the opposite feeder from the one being metered. It would be advisable to momentarily kill the transmitter while the meter is being switched over.

$139 seems like a lot to pay for an rf ammeter, since they have been available at hamfests for as little as a dollar or even less at hamfests ever since the War, but if you look up the price of a quality thermocouple rf ammeter in catalogues from the 20's and 30's, and adjust for to-day's devalued dollarette, you will find the MoFo Junque meter cheap by comparison.  But I would much prefer to use a real thermocouple meter than a rectifier type.

May I suggest silicon carbide rectifiers for when you run out of Schottkys? Zero recovery and 93 cents each.
http://www.cree.com/products/pdf/CSD01060.M.pdf

Probably safer and more legal to use low power and an RF Milliammeter when tuning at the antenna site with remote TX activation! And do it when the band is dead!

Mike WU2D

Ed/KB1HYS Wrote


My understanding is you have two copper plates and a DC path between them made of a conductor with a slight resistance like nichrome NiCr wire. Attached to the nichrome wire you have two wires of dissimilar metals going to the meter.

I can understand the nichrome. That will heat slightly with RF current flow like the electric toaster. The dissimilar wires to the meter isn't as easy for me to understand but I'm going to check into it.

I work with welders all the time. High chrome content carbon steel alloy pipe requires preheat and post welding heat treat. Service companies like Cooperheat and others use two wire thermocouples attached (spot welded) to that pipe to measure and track the heat treatment accurately. I'll see what I can learn from them.

Remember ED your measuring a voltage only on one side of the AC line and that voltage is a result of heating effect on dissimilar metals. The RF is not terminated in the meter. If the RF load is missing you have Don's situation, POOF.

all other things being equal, the RF ammeter is a "tune for max blowsmoke" instrument on any given antenna and load. It's worth noting that AM broadcast stations almost always use a RF ammeter ( Made by Delta) full time in their transmission line.

Considering you can pick them up all day long at hamfests for a buck each, go buy some and see. Just make sure they say "internal thermocouple" somewhere on the meters face, or come with the matching shunt block with matching type and serial numbers.