Hi, This warms my heart. My father was one of the founders of Potomac Instruments and one of the engineers who designed this instrument. This brings back such fond memories. Radio was his passion.

AM radio stations love these guys because when they tune in, their audience triples!

Back in 1975/76 I was an engineer at an AM/FM station in Jeff City with a 4-tower directional night-time array. I don't remember the exact model number, but we had a similar field strength meter. That station had to submit a "Proof-of-Performance" report to the FCC every 3 years. I was the newest, so I got to traipse around central Missouri walking out into farmer's fields "12 yards South of the corner fence-post and 5 yards East of the Oak tree" to take a measurement at least 2 hours after official sunset. I don't remember the exact number, but I think we had 5 stations we were protecting at night. So that would have been 5 radial bearings with at least 3 readings on each bearing. Lots of work in a "proof". I do remember getting a call at home from a Colorado station telling me that our jock had neglected to to the day/night power switch. He knew this because he was listening to our station on his over-the-air monitor. I called the control room, and reamed the guy for not switching from 5KW non-directional to 500W directional. I wanted to fire him because this wasn't the first (nor the last) time he was "too-busy". Alas, one of the sponsor's loved his show.

At least this old 'tech' can always be fixed one way or other. It will not be, "Oh, it doesn't work because we don't have the latest version of software". Keep up the good work!

My first paying job in radio, the summer after I got out of high school, was doing field strength measurements for an AM daytimer. I think I measured about 20 points on each of 11 radials (twice!). A lot of driving and traipsing through farm fields. On the weekend I did a board shift on Saturday and Sunday, as well as took the base current readings.

Neat stuff. Back in 1986 I was the engineer for a two-tower AM in Connecticut, and I would take these readings, then tune the phaser to ensure we were in compliance with our license terms. Great blast from my personal past!

I found a VHF meter at a yard sale years ago. The meter came off it's jewel pivot, and it's battery compartment had corrosion, but it all cleaned up well on the bench. Cost me $200. Also, found several European FSMs from KSL Farnsworth Peak, which we gave to The Old Cable TV Equipment Museum.

I did HF radio for the US GOVT for a number of years. I had a couple of…..🧐…..either one of these sig-strength meters, or one similar to this one. But anyway, worldwide I had dozens of: rhombics, nested rhombics, RLP’s, HLP’s, VLP’s, curtains, high freq CONMONS, low freq CONMONS, quarter wave towers, shunt fed towers, serial fed towers, and so on. It was fun! But, you are correct, as long as you have calibrated records of initial antenna patterns, you can use a $20 pocket signal strength receiver to keep an eagle-eye on tower corrosion and as you say….to “look” at the condition of RF ground systems and radials. This is a wonderful video! Thanks! PS, And oh BTW, I don’t cook my hot dogs with a tower, I plug em in to a wall socket. When the breaker trips, it’s done! A couple of us (like many other goof-offs) back in college days did that for real. I think we found that aluminum nails work best! 🤪

Wow! Blast from the past. Back in the mid-80s I was an assistant Chief Engineer at a AM/FM station in Baltimore. I remember driving around in Pennsylvania in the station remote van with a similar unit testing signal strength when we moved the AM transmitter. Thanks for resurrecting a memory from 40 years ago!

I have some similar instruments (made by Narda and SImpson) that measure 2.450 Ghz in mw/cm2 and is used to check if microwave ovens are leaking. They come with probes and a 5cm styrofoam spacer to get a consistent accurate reading. The crazy thing is, while the instruments I have are decades old, very few places want to calibrate them saying "they are out of support and old" despite them working properly and just needing the calibration. Companies want you to spend thousands on new meters instead, rather than appreciate the quality of these old and very well built instruments.

Back in the 80's when I transitioned from landlines to wireless I often thought of how useful RF glasses would be to look at antenna patterns and such!

Oh my! I had never heard of Potomac Instruments, and it turns out I've lived just down the street from their shop for years!

The perfect receiver for the engineer who's out standing in a field.

I was just a tinkerer at 12-13, but had a Walkman with AM & FM radio. I of course listened to Radio Luxembourg AM 1439 kHz to discover the latest hits, and had to rotate the small unit to get a good (less bad…) signal. Of course, I could only receive this at night (in south Norway), and the reflections shifted from the complex interaction with the ionosphere. So I kept turning the darn thing every minute… I’m not _that_ ‘ancient’ at 58, but this was “media Stone Age”… 😅

The FIM-41 is the big brother that goes up to 5 MHz. That works on some shortwave systems, but was mostly used to certify 2nd and 3rd harmonic compliance, before NRSC cutoff rules required a spectrum analyzer. A major use not mentioned is ground conductivity studies of co- and adjacent channel signals to a potential new AM DA, in regions of the country where rock often makes soil conductivity demonstrably lower than 47 CFR presumptive charts. The nature of FCC designated DA monitor points as license conditions, usually 1-4 miles from a TX site and based on larger studies after new construction or major changes, could be described far better. Also missing from this video is how use of the null perpendicular to the received signal is used to evaluate quality of a potential measurement point. A null less than 10:1 is a horribly disrupted spot, or one with power or phone cables or metal structures distorting the signal levels. Get over 40:1 null at right angles to signal, and that's a really clean point. There's not just one antenna in that meter lid, but two. One is the normal receive antenna, and the other a transmit loop for the calibrate oscillator. The BNC input could also allow use of the FIM as a sensitive detector, such as with a Delta OIB-3 Operating Impedance Bridge. That could be done at increments above and below carrier frequency, trying to adjust for a flat resistive and balanced around zero at carrier reactive impedance curve. Try parsing that calibration certificate again. 1 or 2 percent tolerances.... with so many stacked details as to the basis of the reference fields and their use calibrating meters, that the lumped sum of worst case errors could stack into the 20-40 percent potential inaccuracy range, even if far better precision. Not that I've evern learned to lean slightly, to anchor an elbow in one side, while holding an FIM stable in one hand while fine tuning adjustments with the other.... And then rotate for the null, evaluate point quality, calibrate, rotate back, and measure, and record notes. Lather, rinse, repeat, sometimes out 100-250 miles from another station's site, if doing ground conductivity field studies Rarely past 20 miles in normal proofs.

I went and found the circuit disgram of this unit. This is no ordinary radio…the bloke who designed this was an absolute analog Wizard! I thought…(from “fallout era”) that it wiuld be full of valves, but far from it, almost 80% of the circuit is designed around CA3045 transistor arrays. The only bits where descrete devives sre used is in the audio amp and a j-FET at the RF front end with its fate protected by zeners should someone take a reading right next to a tower with no attenuation swotched in! Another odd feature is it has a Positive Earth, and its rail is -6v that looks to be furnished by four D-cells in series….but the odd thing is, its internal rail is regulated to -6v and the battery can’t be much more than that…so he has used a very low drop out regulator designed around three descrete silicon transistors, two PNP’s in a diffirential configuration and an NPN pass transistor. It is common to see old semicinductor radios built around PNP germanium transistors and using positive earth with these is fairly common practice because then all the transistors’ emitters (the lowest impedance connection on a BJT) can then go to ground by a fairly short path, but this thing has six PNP transistors in it, viz, two in the reg already mentioned, one for the meter and dial lamps, one in the headphone amp and two in the AF amp. all the rest, four descretes and 30 others, five each in six CA3045’s, are all NPN. So the circuit has a strange “upside down” look to it. Looking closer, most of the stages, RF amp, two IF amps, cal oscillator, local oscillator and null circuit are all designed around a diffirential pair with a current sink underneath and a few common emitter gain stages or emitter followers. The RF amp is a very wierd capacitively coupled cascode amp with the drain of the front end j-FET capacitively coupled to the emitter of a subsequent grounded base stage, but has a current sink underneath…..and all this of just 6 volts! So there is barely any headroom to move about. Three things that make this a very different thing from a commercial AM radio are….lack of any AGC, (and this is why you failed to get a good null with the commercialradio…its AGC was working against you), presence of the cal oscillator and null circuit, which a commercial radio lacks…and lastly…designed by an absolute Wizard. The loop antenna of this has TWO active ends that are out of phase, one end goes to the RF amp like in an ordinary radio, but the other end feeds into a common base stage in the null circuit. It is a VERY interesting circuit which looks deceptivey simple st first glance, but the minute you look a little closer you see interesting and mysterious things, and the closer you look, the more you see. One could learn a hell of a lot from this circuit and its clever designer! And that is where the cost is…along with the calibration and lack if drift over time. Ordinary AM radios can have a multitude of evils that a pasted over by the AGC, but here, all is lain bare and has to be spot on, so thanks for making me aware of this marvellous circuit. P.S. If you do go snd look up the circuit, it is drwan like a wiring diagram around the CA 3045 packages which makes in even more confusing to read….as if the positive Earth wasn’t enough….but if you carefully redraw it as diff amps with current sinks underneath and follow the dignal path, that will largely “unscrumple the paper it was drawn on” and allow you to see what a triely clever design it is.

Years ago I knew the Chief Engineer Don Holmes at 10.000 Watt 580KHz CJFX Radio in Antigonish Nova Scotia Canada. I remember seeing him using one of these, I take it, for reports to Industry Canada, DOC in those days. This was many years ago and Mr. Holmes now rests in peace. This Instrument will bring back many more memories. May all the old Chief Engineers rest in peace, and may perpetual light shine upon them. The Reverend Peter Rafuse.

Thank you Geerlings! I have certianly enjoyed my career in RF communications. You are doing a greatjob exposing the next generation to RF and broadcast technologies that they may not have otherwise see. I'm sure you are influencing some in expanding their career options and choices.

I was lucky enough to be able to use one of these back in my radio engineering days in small market radio in the 70's and early 80's. Thank you so much for bringing back those memories!

Stop being so interesting, Jeff and dad! I need to get back to work! Thank you so much for the awesome videos.

When my AM went dark I still have mine, AM is the greatest modulation method for long distance. Long live AM

I have the FIM-41. It has two bands letting you tune up to 5 MHz. This is handy for checking harmonics of the BC station you are working on.

7:42 that's because when you hold an antenna YOU become part of the antenna circuit, your body becomes part of the grounding system! That's a dead giveaway the antenna lacks a good ground leg or you're standing on poor soil/rock for good ground. All antenna in theory are some form of a dipole (like classic rabbit ears) with a positive and negative/ground element that rapidly flip polarity as the radio waves hit them, some of them just "hide" one pole/ear to save space by using different tricks. You can often solve that by adding the right length of wire trimmed to roughly match the frequency used to a ground point somewhere, using say an eye-crimp fitting onto a chassis grounded screw somewhere on the radio. Those are called a "Tiger Tail" in amateur radio and often work well for handhelds or portable setups once you tune the length.

This brought back memories. Around 50 years ago I went out with the engineer with a receiver, but it used tubes and it was much larger and heavier. We put it on our shoulders. We spent the whole morning doing just one lobe. Both of us had our First Phone licenses but I had never worked at a station. It was just as you described. The station was a 3 tower array in the RF ghetto of 1410 KHz. :) I guess we were testing the daytime pattern, but I don't remember for sure.

Certain older pieces of test gear are still considered a valuable must for calibrating particular types of equipment. This is why for instance a particular HP VTVM model is considered more desirable over another.

Ooh, an illuminated 4.5-inch Simpson rectangular style panel meter, with a mirrored scale! I'm in love!

The SONY radio did hit a null on the 1320 station, but the AGC hid the signal strength dip pretty well. Listen closely as the radio is moved around; the interference level goes up in the null.

Also note, manufacturers of sub station and power distribution equipment use a signal strength meter to check for corona. This was at 1020 Khz from what I recall.

note that Fallout took place in the year 2161, but the title is still technically correct because this device will still be working in 2161

I like to take one of my AM radios I have fixed up with me when I go for a walk to see what stations it picks up. If I had one of these field strength meters to take along, I'll bet every passing farm truck would probably have some concern as to what I'm doing 😅

Prefect ending to the video. Both father and son, at the same time: "... see what happens".

I used to work on these (and the FIM-41) when they were still located in Silver Spring, MD

When I lived in Chicago, I was able to watch TV stations over 80 miles away with rabbit ears and bow tie by picking up the bounce off the Sears Tower. I was on the side of the building away from South Bend and toward the building.

This Potomac FSM is built for the MF AM broadcast band. Other versions work on the LW Aviation Navigation band or the SW bands. All on first appearances look identical and all are still in high demand by the radio industry worldwide today as it is one of the few devices around that can give a accurate calibrated repeatable numerical value to the received RF signal strength of a selected radio station, thus the high price.

Thanks for linking to the loop antenna in the video description! 👍

Awesome as always but what really struck me was the joke about magic glasses that allow you to see RF. I remember day dreaming about that early in my RF nerd hobby 30 years ago. I don't think I've ever heard someone else mention the same idea. Made me smile.

It becomes even more charming to hear the discussion of this great old gear when the guest has a regularly Mr.Roger's-sounding voice!

My father w89job spent many of his retirement years going out and doing field strength readings with various equipment. I'm pretty sure I saw a lot of these in his stash.

Great explainer! And I will admit to that curiosity, along the lines of a 1240 khz "grilled cheese." 😉

Used many times. I do not miss the days of NRSC proofing. Retirement is such a lovely thing.

We have a guy that drives around once a year or so and gives us reports on our signals. I am not sure who it is or what equipment he uses but the report is quite detailed. As for the "glasses" Ubiquiti makes a spectrum analyzer wifiman wizard that lets you "see" the RF as it maps the room/building you are in. You need an iphone with the lidar on it to take full advantage of it but it's a neat tool. Also laughs in 80 year old grounds sitting on rock basically for my AM tower. Chasing RF was fun till I went nuts with new grounds.

I remember doing engineering proofs with that very same model back in the early '90s. Had the cops stop us a couple of times atoask WTF we were doing, when we were checking the night pattern ...

You should try an active loop for HF RX; broad bandwidth with no tuning.

Way back before I retired I used a Potomac instruments fim 41 to measure am and FM radio station signal strength. Particularly am directionals where we had to make sure that in a multitower array that no was deep enough to meet FCC requirements for protecting adjacent and Coach channels a distance away. These were highly calibrated and accurate receivers with accurate metering.

I have many different boxes for field strength readings, but most are for aligning the receiver antenna for best reception. They work on VHF, and UHF TV and also FM radio bands, as they are fun to use to do antenna installs, kind of like the units for Satellite TV dishes.

That is one of the most expensive radios I have ever seen, and I used to be a meteorologist many years ago. We had calibrated RF "radio" equipment used in an RF chamber to measure both conducted and radiated emissions. You have the most expensive hotdog cooker ever! As a youngster I used to cook hotdogs with a board, a couple of bent nails, and a killer cord! Those were the days!

Plenty of time spent cradling this thing doing monthly monitor point readings, partial, and full proofs. So glad I don't do that any longer!

These are great videos with your dad. I have the original highly sought after SDR USB stick. Bought it for like $30 maybe 10 years ago? A year later guys were selling their used ones on Ebay for up to $200 and they weren't making that version anymore. I can't remember why they were sought after anymore. I'm planning on breaking it out again after I move.

Excited to hear about you getting into SDR, looking forward to some videos potentially? (:

I wish I had a buck for every time I jumped over a farmers fence, into a muddy field with a Potomac FIM-21 to get a measurement for a directional antenna proof of performance.🤓

In the early 60's I read in an electronics magazine that if you held an AM radio with a ferrite antenna upside down you would lose the signal. I swear at that age it did work as they said. This may have been the April Fools edition. Like the Black Noise Generator made out of a black bean soup can. Unfortunately, I haven't seen an AM portable radio in so long to try it. Yes, I do have a ghost detector.

i am very impressed with the loop antennas! i have one myself and made a patch cable to hook to my IC-7300 just to experiment with, and the results were surprisingly very effective on picking up week signals. i have also used it on a couple other radios and they improved signal alot! 73's KF0PRI

Used one in the mid 70's Pattern checking AM broadcast station. That's a precision instrument.

The successor to this is the Potomac Instruments FIM-4100 which is a modern software controlled digital readout instrument which is really great to use. Problem is the high price. Over 14 grand. It makes the analog FIM 21 and 41 look pretty good. Although no longer manufactured used ones can be found on used equipment sites for 2 grand or less. Calibrating the older analog ones is costly and Potomac Instruments is the only company that will certify the calibration.

I'm now calling it official. Your father is now Purple Shirt Geerling. Blue and Red make Purple. I also notice that your dad through challenge down, and you blanked it. Pi based RF detecting glasses.

All terrestrial radio costs too much money! But you have to love those still involved in this technology.

Being able to accurately compare antenna's signal strength is nice to have. I've gone down the rabbit hole and now own way too many meters. With TV signal strength meters, ones that can measure ATSC and QAM, they will show you the bit error rates and constellation. You can get a really good idea of he health of your antenna and amplifier setup. Gone are the days of being able to see ghosting and intermod on analog TV.

This reminds me of my Grandfather. While he was living in northern Ontario Canada around the mid 30's. He would turn his AM radio on after dinner and sit in the corner so he could see up the road for oncoming headlights. He decided not to pay the radio license from the hardware store, so if he saw headlights from a vehicle, he would shut the radio off. The bylaw was enforced by a vehicle that had a directional antenna that could detect the intermediate frequency from the radios when they were turned on. Imagine a knock on your door and someone asking if you have a license for that radio!.☺

I worked for the Civil Aviation Authority here in Australia, we used to use the Potomac to quantify the output from our Non Directional Beacons (NDBs).

All this and no poking it with a hotdog? Please make that a running gag for these videos with your dad scolding you for it.

I bought a small digital-ready television in December of 2007 in anticipation of broadcast tv going digital in 2008. I have yet to take it out of the box. AM and FM radio are all I have been into all this time, plus some KZfaq, obviously, on my android. Some one of these years I will get around to unpacking that television. I sure hope it works, because forget about the warranty. I often eat out by myself, so I sit at the bar. I look at all those television screens and realize I'm not missing anything.

When I worked Telstra broadcasting directorate here in Australia one of our crews lost the calibration dipole that came with one of our Potomac UHF FIM’s whilst doing a survey . The headache and cost of getting a replacement including sending the unit back to the states to get a replacement calibrated to it. Fortunately a farmer found the antenna and managed to get it back to us so the company paid for a brand new TV antenna installation at his house as a reward.

Oh the classic Potomac field strength meters. I still see them on eBay for an arm and leg.

By the way, the loop that you are using had a measured Q of about 800, when we measured it. Works well, and cheap.

Seems like it's nice to have a father.

I have no idea or knowledge of any of what is talked about in this video. I'm here because I enjoy to see a son doing stuff with his father, that is so rare today. And who knows I might learn a thing or two :D

This is a Potomac FIM41 field intensity meter. It's basically a calibrated, precision receiver used for measuring radiated signal strength from a AM transmitting antenna for proof-of=performance purposes.

The more I learn about AM radio the more it reminds me of how EM surveys work for exploration geophysics. Even everything having a specific orientation and going "null" if its 180 degrees out. Unlike radio though we have 3 "antennas" now for X Y and Z measuring the strength of the signal in 3 orientations allows a 3D "image" so it can be a pain in the ass to get lined up correctly. I cant help but think a random geo was screwing around with an old AM radio in the bush back in the day and figured out that theyd sometimes get a signal with the antenna "pointed" at the ground. These AM field strength testers remind me a lot of the very original looking 2D EM recievers from way before I was born.

Never in my life did I ever expect a KZfaq channel to create internet 'sizzle' around a Potomac FIM. I'll admit to being slightly annoyed by this.

A little multipath on FM... I used to have to take FI readings when I was out at KRVN. A lot of monitor points both day and night patterns. Great video guys!

We use FIMs in the wireless industry to determine if the detuning on wireless communications sites is working, not messing with the AM station pattern.

What about FM? I don't mess with that newfangled sorcery.

Your dad and you share the same timbre and cadence. Thats really cool.

Cool collectible radio, however I can do the same thing with my cheap AM pocket radio by turning it. I know where the null is, so I know where the transmitter is and my signal strength meter is my ears. I do really like that tunable external loop antenna, more than the radio. I might build one of my own. Have been meaning to do it some day. You can also enhance reception of a radio by putting it near the loop even without a connection.

Guys, this is right up my alley! But I´m a noob, here in Germany AM is dead, exept for some Turkish and Chinese stations. Could You do an antenna giude for dummies who don´t have any sphisticated gear like this. Btw it´s very cool that the company is still around to calibrate it. I´d love to just scroll thru the scale for some Johnny Cash... Yeah, no, I´d love to get by with the least financial expense- so to say. I collect old tube radios, still use my FM receiver and cannot imagine a future without analogue radio. Kind regards

Similar devices were used in cable TV distribution systems as well at higher frequencies of course, calibrated at dbm and dbmv, by companies such as Wavetek and Texscan,

Hmm , a feild stregth meter didn’t used to cost that much years ago . I used mine to match antenna to radio RX/ RT two way system . $10 or $20 or so . Seems a bit excessive to me . Because the operator in any case has to calibrate the device himself any way . But ok , what ever .

I'm surprised that there are so few field strength meters in the U.S. given that theres almost 5K AM broadcsat stations and ~6K FM stations. Do the stations share them amongst each other?

Another VERY VERY interesting video! Loooove this content! :)

FYI. The loop can be made to have extended range, by just changing the internal capacitor.

The best place to take directional array monitoring point readings is standing next to the marker.

I believe it's a Sony ICF-506. I have the ICF-19. They're almost the same radio except the ICF-19 does not have an AC power supply and it uses D cells instead of AA. The ferrite loop is not vertical, the null just isn't as sharp as your Dad is expecting. If you take it outside in the open, you would probably get a sharper null.

A thing of beauty and a joy for ever, it would be very Fallout-y if it was totally tubular :)

a spectrum analyzer used for the same purpose today is much more expensive, maybe 10-20 (but ofc does many more things) calibrated antenna alone will run you more than the entire device AM demod option will be a few thousand, too, if you don't just want to look at the signal, but also listen and do more specialized analysis stuff

ive have a old williams shortwave radio from world war 2 its bigger than a microwave and a working one cost 1800$usd i love that thing

Nice!!!!!!!!!!! No usagi-superlative-blabla, no adrians-digital-repetition ... just to the point! Love that channel! Perfect! ❤

Very cool vid. First time seeing a field-strength meter!

I used one a lot for AM tower monitor points.

Good Day. Very cool and educational. Thank You Both & Best Regards.

Similar a fricking 50ct screw costs $500, because it's FAA aircraft certified or something.

That's the unit the FCC guy had when checking my Part 15 station. Also our local KELO-AM is 1320. KMOX being 1120 I wonder what 1110 KFAB looks like from your area as KFAB is also 50,000 watts.

Potomac Instruments are still in business and moved to Frederick

I'm tempted to try mapping my local repeaters or even my own ham station (would need a way to key it remotely) using a TinySA and a loop antenna.

Very cool. I listen to shortwave on an SDRPlay RSP1A and use SDRconnect software on Linux. A decent cheap antenna is MLA-30+.

You should touch the tower with a antenna on a radio and see what happens! You could try a bunch of different brands.

Seems like you two have got your calibration set as well.

How about touching a four foot fluorescent bulb to the tower?

How about doing a video on the Narda or similar emi/emf probes. I've got a cased set, fully operational in my garage. These are indestructible (unless you deliberately fry them) and easily serviced. Various heads work from (almost) DC to (almost) light. Smaller commercial versions are also useful for all EMI and EMC work, and including ESD.

Let me see if I got this right... Directional antennas Are directional, ANd field strength meters actually measure field strength... If they are calibrated. Good to know.

AM, Ancient Modulation. Except for Air Traffic Control.