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[Articles Home]  [Add Article]  

A Handy-Dandy Super Cheap Field Meter

from Eric P. Nichols, KL7AJ on July 2, 2017
View comments about this article!

"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 03/04/2008





A Handy-Dandy Super Cheap Field Meter

by

Eric P. Nichols, KL7AJ

One of the most useful instruments one can have around the ham shack is a simple R.F. field strength meter. Most of the time we aren't looking for National Bureau of Standards accuracy, we just want a quick and simple method of tuning our system for "maximum smoke." The field strength meter I describe is just such a device; so simple and cheap, no ham should be without one, and sensitive enough to be truly useful. It will give a useable indication or relative field strength at power levels from QRP to full legal output.

You can deploy this instrument just about anywhere. You can just set the thing on your operating position, or if you really want to get picky, you can put it on a pedestal below your dipole and use a pair of binoculars to read the thing.

There is no sensitivity adjustment pot included, to make things even simpler. Sensitivity is adjusted by tweaking the length of a collapsible antenna.

The circuit is extremely simple, as you can see by the schematic. The most expensive part is the microammeter. (The last time I went to our local Radio Shack and asked for a microammeter, the genius behind the counter came back with some sort of battery tester and said, "This is the smallest ammeter we've got." Needless to say, you will have to find a more apt source for your microammeter, such as Ye Olde Junque Box). The real "guts" of the device is the MPF 102 J-FET which acts as an R.F. detector and DC amplifier. Incidentally, this beastie is so sensitive, it will detect DC fields as well, such as approaching lightning storms….a handy little side-effect you should be aware of.

You can install the whole shooting match inside a small plastic project box, or you can build a box out of double clad circuit board, which gives you that rich coppery appearance. Mount the collapsible antenna securely to the top of the box. (You might want to add some dead weight to the bottom of your box, so the thing wont fall over with the antenna fully extended).

This device is also handy for tracking down excessive R.F. in the shack. Theoretically, you should have more R.F. near your actual antenna than you do in your shack…if not, probably some investigation is in order!

By the way, you may find that the D.C. field strength capabilities of this are even more interesting than the r.f. aspect. A slight modification of the circuit, replacing the junction FET with a MOSFET, will make this a true static field instrument. (A J-FET requires a finite, if minuscule amount of gate current, while a MOSFET does not). The MOSFET version, with a long, vertical wire antenna, will also allow you to investigate phenomena such as electron precipitation events, if you happen to live in high latitudes. Electron precipitation is one of the major cause of sudden R.F. blackouts…and can be easily read with such simple instruments.

You will find this handy little instrument useful, educational, and fun. It's a great weapon in your radio arsenal.

73 de KL7AJ


Member Comments:
This article has expired. No more comments may be added.
 
A Handy-Dandy Super Cheap Field Meter  
by AK4YH on July 2, 2017 Mail this to a friend!
Hello, what should the range of the microammeter be?
Gil.
 
RE: A Handy-Dandy Super Cheap Field Meter  
by KG7HMT on July 2, 2017 Mail this to a friend!
It is a 0-100 microamp range meter.
 
K.I.S.S.  
by VE3CUI on July 2, 2017 Mail this to a friend!
A practical application of "K.I.S.S." in Ham radio, as in, "Keep It Simple, Stupid!"

I like that.
 
RE: K.I.S.S.  
by K6AER on July 2, 2017 Mail this to a friend!
The circuit has no dc rectification of RF and an adjustment for sensitivity. On/Off switch would be nice.
 
RE: K.I.S.S.  
by G4AON on July 2, 2017 Mail this to a friend!
I don't agree with the statement that a simple field strength meter is useful around the shack. I have a lot of test gear and build some of my own gear too, I haven't a field strength meter and don't know anyone with one.

Am I missing something? I don't think so.

73 Dave
 
RE: K.I.S.S.  
by K6AER on July 2, 2017 Mail this to a friend!
Let me expand on the RF rectification part. The circuit will amplify any AC component of the antenna field and that component will be placed across the meter as well. Only the frequency bandwidth of the FET is the limiting frequency factor. The AC component will dampen the meter movement. The meter needs a .01 across it. The input needs a diode and a choke in order to make the circuit sensitive to RF and to provide negative input DC cancellation.
 
RE: K.I.S.S.  
by KL0S on July 2, 2017 Mail this to a friend!
Well, a FS meter does have some utility at the workbench albeit not routinely. But I used mine just recently to qualitatively demonstrate the difference between a 2m HT with the rubber duck and the same antenna with a 1/4 wavelength single "radial" attached; as you'd expect the FS was increased when measured at the same distance with the wire attached. Sure, not a quantitative measurement but when you're simply looking for a delta one way or the other it's handy. BTW, mine is part of the original SWR meter I first used as a Novice in 1969 - it has a detachable collapsible whip when used in that mode.
 
RE: K.I.S.S.  
by KB1GMX on July 2, 2017 Mail this to a friend!
Usually a field strength meter is most useful for tuning an antenna as stated for max smoke....

Once tuned then the mismatch has to be resolved where it exists. With electrically short antennas (mobile whips)
it can be the other instrument that tells when max radiation is occurring rather than acceptable match which for short or otherwise compacted antennas is not always coincident.

I do have several. The oldest is a lafayette tunable unit.


Allison
 
RE: K.I.S.S.  
by K9MHZ on July 2, 2017 Mail this to a friend!
Guys, remember this is a rerun article. Eric might not even know this is up again. I remember some food fighting the first time it was posted.
 
Field strength meter... pretty handy...  
by K6BRN on July 2, 2017 Mail this to a friend!
Field strength meters are pretty handy for sniffing out larger RF "leaks" in the shack. Just a little bit ago, I used one to identify an RF problem in a friend's shack. Lots of RF in and around his bench, with plenty of nearby electronics unhappy about it. Turned out to be a swap-meet purchased coax jumper from China that was bad. When we cut it open, there was just a few wisps of braid inside - maybe 35-40% coverage. Swapped the jumper out and all was well. I've also been able to sweep my shack for RF leaks, and after finding a few unexpected ones on digital and power interface cables (to power meters), inserted some badly needed toriods to suppress them.

So... they are useful, and now you know someone who has one - me. BTW - I also have an active RF sniffer with frequency counter and bar graph signal strength meter. But this unit was TOO sensitive to pinpoint the leaks I found with the passive field strength meter
 
RE: Field strength meter... pretty handy...  
by KB6QXM on July 3, 2017 Mail this to a friend!
One thing I have learned with the many years as a ham. MOST hams are CHEAP!

Of course, this will require will the ham to build it and I do not know how many of the appliance operators will actually build it
 
Appliance Operators?  
by K6BRN on July 3, 2017 Mail this to a friend!
Bobby (KB6QXM):

Boy. Am I tired of hearing the label "Appliance Operator" thrown around with so much contempt. Why so much disdain for people who enjoy this hobby without being up to their necks in detailed electronics construction? This is AMATEUR radio. Loads of ways to enjoy it, including contesting and rag-chews. Not everyone can participate in the detailed technical end. Not everybody should. Does that make them lesser persons? If so, maybe all radio amateurs should be professional comms engineers with at least a Masters degree in EE. And be millionaires, too, so they are not so cheap. Oh, wait... then it would be Professional Radio. Or Millionaire Radio. Pardon me.
 
RE: K.I.S.S.  
by PBJOK on July 4, 2017 Mail this to a friend!
"Am I missing something? I don't think so."

Yes. A brain. Who cares what you think? This was a nice KISS article.
 
RE: Field strength meter... pretty handy...  
by PBJOK on July 4, 2017 Mail this to a friend!
"Appliance operators?"

You must be one of those [many] hams who smell bad and have trouble uttering complete sentences - yet you look down at "appliance operators." Snobby snobby snobby. My kid accompanied me to one hamfest some years ago, encountered a handful of people like you and said "Nope!"

He is a ham but wants nothing to do with the old crowd. I don't blame him a bit.
 
RE: Field strength meter... pretty handy...  
by SWMAN on July 4, 2017 Mail this to a friend!
Wow, I thought this article was suppose to be about building a nice simple field strength meter ???
 
A Handy-Dandy Super Cheap Field Meter  
by AC7A on July 4, 2017 Mail this to a friend!
I don't think the JFET is being applied as a linear amplifier as shown. The JFET is a depletion mode FET, and the drain-to-source current (Ids) is controlled by the voltage applied between the gate and source (Vgs).

In the case of an N-channel JFET the gate is normally biased negative relative to the source. Commonly, that is achieved by a resistor connected from the source to ground. The voltage drop developed across the source resistor biases the source positive relative to the gate, thus the gate is more negative. Also, a resistor is connected from the gate to ground and serves as a gate current (Ig)return. In the case of this circuit, the gate is floating and there isn't any predictable Vgs.

If the FET were linear biased the voltage gain at the drain would be the transconductance (gm), times RL. The latter being primarily the load presented by the microamp meter resistance.

I suspect that in this circuit rectification is taking place at the gate-to-source, or gate-to-drain junctions and the RF rectified current. That is the current being measured by the microamp meter. If the FET were operating in linear mode some current would be indicated by the micrometer without any RF present. That current would be associated with the dc operating current (Id)as established JFET's electrical characteristics.

Have great Independence Day, and get on the air if you have the chance!

Thomas - ac7a
 
RE: A Handy-Dandy Super Cheap Field Meter  
by KB6QXM on July 4, 2017 Mail this to a friend!
@K6BRN,

I do apologize if I triggered you. I will give you a little perspective from why I have used the term Appliance operators. When I was introduced to the hobby back in 1971, there was much more home brew. Hams knew how to build amplifiers, solder coax, build radios (remember Heathkit). Hams had to go to their local FCC office and take the tests. Not multiple choice tests with published questions. They had to know code. Send and Receive. The Novice test was a 1 year non-renewable license, so either you upgraded or you found another hobby. If you said you were a ham in those days, it meant something. The bands were not saturated with Radio Sport people. People actually had real conversations and rag-chewing was OK. I have and continue to have a technical professional career in the Silicon Valley. I was in a local ham club meeting sitting next to a newly-minted Technician class who said "what is a repeater" and did not know how to solder a PL-259 onto a piece of coax. Are all hams Appliance operators? No of course not. Just ham radio has changed significantly since my introduction. Oh, did I forget to mention the difficulty of putting up an antenna because of HOAs and CC&Rs? I wish ham radio luck in the next 20-30 years. Lastly, my name is not Bobby. 73
 
RE: A Handy-Dandy Super Cheap Field Meter  
by SWMAN on July 4, 2017 Mail this to a friend!
QXM - Well said, you took the words right out of my mouth. 73
 
RE: A Handy-Dandy Super Cheap Field Meter  
by AF6AU on July 5, 2017 Mail this to a friend!
Geeze, we went from a simple Field Strength Meter to bemoaning Newbies.... O.M.G. If you came across a Tech that was that naieve, did you do the right thing and TEACH him/her, sharing in all that knowledge you have stored in that 4-1000A of a skull, or did you chuckle, smile, and walk away?

I may build on of these, at least to play with. My issue is a 10Kw AM boradcast station 4 miles away, so Tweaking with an input tunable rejecting L/C circuit to kill the BCB signal will be needed.
 
A Handy-Dandy Super Cheap Field Meter  
by KE8HAG on July 5, 2017 Mail this to a friend!
I just recently became a Amateur Radio Operator (HAM) and as a new operator, I'm becoming to wonder if I really want to be a HAM.

What I see? I see many hams wanting to help and do help promote the hobby and I see others who belittle most everyone junior to themselves. I see a lot of resentment from enough of the "old timers" to negatively effect anyone wanting to become a ham. I see on numerous forums, mounds of insults, name calling and just short of, call someone just plain stupid. Then there are a good number of operators who really doesn't care which way the hobby goes. More so, I see operators that think they are still in the 11 meter band, no rules hehaw. The FCC just about has to be held at gun point to act upon it's own rules and law's.

I think the meaning of being a ham has become so segregated that one is almost forced to choose what part of the HAM world do you want to be part of.

As far as it goes for the subject of this post "A Handy-Dandy Super Cheap Field Meter". I think it's great to share this kind information for those just getting started,It's a stepping stone.
 
RE: A Handy-Dandy Super Cheap Field Meter  
by SWMAN on July 5, 2017 Mail this to a friend!
HAG, Also well said.
 
RE: A Handy-Dandy Super Cheap Field Meter  
by KB6QXM on July 5, 2017 Mail this to a friend!
@af6au,

What is it about people in 6 land getting triggered and name calling?
Calling me a 4-1000A head. I own a 4-1000A, so I indeed know what that is or calling me a name that is not even mine. Did I name call?

I simply put what my experience was for a previous statement of why I used the term Appliance Operator. Did my statement ask for you to flame on me and call me names? I simply based my observations in the differences in ham radio from the early 70s to what it has become.

Calm down.

73
 
RE: K.I.S.S.  
by N1LO on July 5, 2017 Mail this to a friend!
Yep, KL0S and I used a passive field strength meter to do what I have dubbed the 'field strength demonstration' for the attachment of a counterpoise wire to a 2m HT. For this demonstration you definitely want a sensitivity control pot for setting the base reading to a specific meter reading and for compensating for the distance between the HT and field strength meter and the HT's power level.

I have a very old passive field strength meter that was given to me by an Elmer and I retrofitted the antenna connection and changed it to a panel Mount BNC Jack.

I can direct connect small HT sized antennas or external sensing antennas using a piece of coax for simple transmitter checks and relative field strength comparisons. You can also connect a step attenuator between the antenna and field strength meter to measure differences in field strength in decibels.

I recently used my field strength meter as an RF milliammeter while tuning the reject rods on the duplexers of our local repeater. I connected my HT to the input Port of the duplexer and transmitted with five watts on the frequency to be rejected and had my dummy load on the output with a tee. I connected the field strength meter to the branch of the Tee. It was very easy to detect the null when the reject rod was perfectly tuned.

So the field strength meter made an adequate substitution for an expensive RF milliammeter (which I don't have).

My passive field strength meter was barely sensitive enough to do the job and I am interested in perhaps building an active one. I would definitely want a sensitivity control
 
A Handy-Dandy Super Cheap Field Meter  
by KE4ZHN on July 6, 2017 Mail this to a friend!
No matter what anyone posts here, it always becomes a pissing match. Sad. I thought hams were supposed to enjoy communicating, not ripping each other a new ass at every opportunity.
 
RE: A Handy-Dandy Super Cheap Field Meter  
by AL7PQ on July 6, 2017 Mail this to a friend!
Hey man, just ignore the negativity. Don't let a few loud people drive you away. To look through this thread you would think ham radio people are a bunch of jerks...but it's not that way. There are lots of us who enjoy the fascination and yes, the support. There are lots of people, like Eric, who spend their time helping others. I have great appreciation for him and others. The good people in the hobby aren't making a lot of noise, so you might not see them at first glance. I wish you much enjoyment in the ham radio community.
 
RE: A Handy-Dandy Super Cheap Field Meter  
by K9MHZ on July 9, 2017 Mail this to a friend!
"What I see? I see many hams wanting to help and do help promote the hobby and I see others who belittle most everyone junior to themselves. I see a lot of resentment from enough of the "old timers" to negatively effect anyone wanting to become a ham. I see on numerous forums, mounds of insults, name calling and just short of, call someone just plain stupid. Then there are a good number of operators who really doesn't care which way the hobby goes. More so, I see operators that think they are still in the 11 meter band, no rules hehaw. The FCC just about has to be held at gun point to act upon it's own rules and law's."


What I see? I see many new hams posting very generalized BS about a hobby populated with some very talented, good and decent people. Just because you showed up and took a test doesn't suddenly give you vast insight, OM.
 
RE: A Handy-Dandy Super Cheap Field Meter  
by KE8HAG on July 9, 2017 Mail this to a friend!
What I see? I see many new hams posting very generalized BS about a hobby populated with some very talented, good and decent people. Just because you showed up and took a test doesn't suddenly give you vast insight, OM.

Sorry you interpreted the post as BS. Observations stated in the post were first hand observations, not vast insight. In fact, no insight at all. Undeniably, there are some very talented, good and decent people in this hobby. Sorry that you were offended.
 
A Handy-Dandy Super Cheap Field Meter  
by KA4NMA on July 14, 2017 Mail this to a friend!
I've always wanted a FS meter for hf and vhf. I'm disabled and cannot build one. Can somebody help me out?
Randy Ka4nma
 
RE: A Handy-Dandy Super Cheap Field Meter  
by W6EM on July 26, 2017 Mail this to a friend!
Randy: Not able to send you one as I'm not at my QTH and won't be for a while.

Take a look on EBay. "field strength meter" will bring up a long list of mostly older CB SWR/FS meter combinations. Anywhere from $10 to $30. Just make sure that it has the small, screw-in collapsible whip antenna.

They will be sensitive enough that any HF signal above 5 watts into an antenna will be detectable and probably true at VHF as well. Consist of small diode rectifier and bypass capacitor ahead of a sensitive (100uA to 1mA) meter and a line coupler for SWR measurements. Both in one device.

Besides your needs, an untuned and unbypassed meter amp could really curl the meter needle, in a strong field. And, if memory is not mistaken, any reasonable on state current will be more than 100 uA for an MPF-102. Although, I might be mistaken. Hmm. When I get back home, I might wire it up with a larger meter, as in a 1mA at least.

73.

Lee
W6EM
 
RE: A Handy-Dandy Super Cheap Field Meter  
by W6EM on July 26, 2017 Mail this to a friend!
I just looked up what the 0VDC Vgs drain current range for an MPF-102 typically is. Per the On Semi datasheet, IDS runs from 2 to 20mA. So, realizing that 9V is less than the 15V supply in the datasheet, it still presents a problem. A midscale or higher DC current indication with no RF or AC signal whatsoever.

This could be overcome with a source resistor so that a few mA will bias the gate negative, thus turning the device off. But, no such animal in the diagram. And, the results would not yield a linear response from the meter, even if you could cut of the no-signal DS current.

Don't risk curling a good 100uA meter pointer. Try at least a 25mA FS meter with this circuit and get used to a no-signal, mid scale DC value...... Not exactly something most of us would want.

 
A Handy-Dandy Super Cheap Field Meter  
by N8CMQ on July 28, 2017 Mail this to a friend!
When I need a quick RF field strength meter, I clip a diode into the leads of my Fluke DVM and use that.
If I need more than that, I build it up with junk box parts.

N8CMQ Jeff
 
RE: A Handy-Dandy Super Cheap Field Meter  
by N9MKF on August 22, 2017 Mail this to a friend!
are these the same old fossils/has bins that were around 50 years ago. sounds like it to me............
 
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