Being a bit of an antenna fan, and finding myself yomping up hills with bits of wire, PVC pipe, tape measures and all the other paraphernalia we seem to press into service for getting our signals out, I thought it might be time to get myself a field strength meter. I want to homebrew it and make it nice and compact to fit into the SOTA bag. I’ve had a scout 'round on the net and found a couple of designs, but they mostly seem to active (I’d quite like a passive FSM, although its not the end of the world if not) and seem to stipulate that they are designed for VHF. I’m wanting to use mine for HF too.
To this end I had a look through the pdf manual for the MFJ-802 and it seems to me to be nothing more than a germanium bridge rectifier with a couple of telescopic whips on the input and a .1mic capacitor and a milliamp meter biased with a potentiometer across the output. Even I can do that! And for lots less the 50 quid.
Am I over-simplifing things? Can I just lash one up out of bits? Does anyone know of any more schematics kicking around? All thoughts and comments welcome… =)
A Field Strength Meter is not less than a very well calibrated voltmeter !!!
Now, you need to know how well accurate you want it to be
The antenna of your FSM can be seen as a transducer with what we call an “antenna factor” (= ratio of the electric field strength E of a plane wave to the induced voltage V at its nominal impedance - 50 Ohms here - Ke = E/V). Calculating this factor with frequency is well explained in literature, but you can use the well known CCRM formula Ke = 20 log (F/40) with F=frequency in MHz (only for dipoles and monopoles).
As the definition mention it ( PLANE WAVE ) means that you must be in far field form you source !!! (it’s very important - lots of people forget that)
The FSM or test receiver is your well calibrated voltmeter. Why not using your S-meter ? But don’t forget to desactivate your AGC, and it must of course to be calibrated (generator) over frequency…
The well calibrated voltmeter… that is all about… Have look at a well known professional test receivers (gray and blue )
The MFJ looks to be a fairly standard design for a field strength
meter.You can think of it as an untuned Crystal Set with a meter
substituted for the headphones. The only reason that they may be
considered as VHF meters is that although they are untuned, the pair
of telescopic whips would act as a VHF dipole, but would be far less
efficient for HF. However, this is not really important, as you would
not be looking for an absolute reading but a comparative one, e.g. if
you were adjusting an antenna you could set the FSM for mid scale,
leave it in position, and having done your adjustments see if the
reading showed an increase or decrease in reading. Or you could, if
practical, walk in a circle at a distance from a horizontal antenna to
get some idea of its radiation pattern. Certainly not a device which
will in any way analyse your antenna, and I would not think of much use
on a SOTA activation. Easy to build though, - a plastic box will do if
you haven’t got a metal one to hand, and use a low reading meter in the
circuit for best sensitivity. Hope this is of use to you, and good luck.
Excellent, thats lots of use to me. You have confirmed my suspicions (or allayed my fears) on the subject. I was already plowing ahead with it, having ordered myself 5 of the same germanium diodes used in the MFJ design. My plan is to build one up minus the meter, then use my digital multimeter as the output initially. A couple of readings and applying Ohm’s law should help me figure out what analogue meter to keep an eye out for.
I don’t need anything calibrated or even need to use it to do anything in the way of analysis as such (I am the lucky owner of an MFJ-259B analyzer - who says money can’t buy happiness! ;), other than getting a feel for radiation patterns, F/B ratios and the like.
I’m not entirely sure that money can buy happiness, but it sure would
be nice to be miserable in luxury, - like having one of those analysers!!
I would just use the lowest full scale deflection meter you can spare,
after all you can reduce its sensitivity with the pot, but there isn’t
any way of increasing it, or for real luxury, you could buy one of those
cheap yellow digital multimeters off eBay, and build it into the unit.
Mentioning F/B ratios suggests you might be considering using it with
a beam however, and I think your best bet for that would be to get
another station to send you a steady carrier, and rotate the beam plotting
your ‘S’ Meter readings. The advantage of this would be that you should be
able to get the value in dB per ‘S’ unit from your rig’s spec, and plot the polar diagram directly in dB.
Good luck with it anyway, and look forward to working you soon
You are quite right about an analyzer being a luxury. In a way, it almost “spoils” you to have so much information so readily available, but for me its been a learning tool as much as a “tool tool” - my math has come on leaps and bounds since Christmas. Being one of those poverty-stricken-student-types I find a great deal of pleasure doing things on the cheap (thus the FSM), but a good set of tools around you makes it a lot easier to do a good job of a cheap job.
I have just been lay sleeplessly in bed, and thoughts of FSM’s wondered into my head - I think the yellow multimeter idea is brilliant. It is as you put it “luxury” and its as cheap a way of doing it as any! Just wish my YL’s house had a well-furnished junkbox and I’d be away =)
The specific job I had in mind (or at least, the job that got me determined to build an FSM) is setting up a G4MH minibeam for best F/B ratio. Incidentally, I’m not necessarily expecting miracles from said antenna - its just something that Geoff M3SFN has found for cheap, is fun to have a fiddle with and might even fit in the back garden of next year’s student house! It seems that an analyzer and an FSM are the tools of choice to prune it up. Almost all of my operating in Bangor is /P from a local high spot (save for a bit of 20m from the “home” QTH), so as such the hill is my shack, which would explain the FSM’s addition to my already bulging SOTA bag =)
Your method of using the S-Meter to figure out the pattern of a beam is a great, I’m going to stick that one on the “cranial hard disk” =). It reminds me of an article I read where somebody drove around with an FT-817 whose s meter had been calibrated in a lab and used it to plot the coverage area of a repeater.
This thread reminds me of when, as a new G8 in 1965, I had just built a more powerful 70cm tripler-amplifier using QQV06-40A’s and wanted to know if the maximum output corresponded to maximum dip on the anodes. I had a 100 microamp meter handy in the junk box, so I put a crude wire 70cm dipole on the terminals and a diode across them, and stuck the thing on the garage roof at the end of the garden. On pointing the beam at the garage I was able to tune up the rig whilst watching the meter through a spotting scope, and I also checked out the polar diagram of the beam. It worked extremely well!
The point here is that your original idea does work, and crude and simple is as good as anything! Try a lashup before building something more permanent.
Sorry they’re a bit late, but my comments as follows:
Antenna analyser - wonderful. I bought one recently and it’s revolutionised my antenna building - it’s quite an education actually quantitively determining the effects of changes on a number of parameters. Don’t know how I ever did without one!
Yellow digital meter - brilliant as a detector in an FSM/Wavemeter, very high input impedance so little loading and not so prone to sudden death if you overload it as a 50 or 100 microamp meter!
Antenna tests - to get meaningful results you really need to be taking measurements in the far field - for practical antennas 2-3 wavelengths away is usually OK (although for very large arrays it can be more) this means that an untuned FSM might struggle to indicate at low powers you might want to use for this sort of test. For HF I have used a tuned version - basically just an absorption wavemeter with a short whip antenna coupled to the coil - a lot more sensitive. You might need to talk nicely to Geoff of course and locate him in the far field too with an HT to report back what the meter’s saying…
For something more sophisticated but simple, I’ve used a radio with the AGC disabled and the output fed into an audio wattmeter (Marconi TF 893A - not exactly portable!) to get relative readings of signal strength - but check that for the radio you are using the range of signal levels you are working over is linear.
Have fun with the building and experiments, it’s what it’s all about!
