Determining the internal resistance of a meter..

This from a .pdf document on meter shunts posted on the internet

Clearly, though, you need to know the resistance of the meter itself in order to work out the value of the shunt you're going to need. Luckily the internal resistance of most meters is specified when you buy them new.

For example the resistance of the MU-45 type 0-1mA meter movement sold by Electus (QP-5010) is specified as 200 ohms, while the more sensitive 0-50uA model (QP-5012) has a specified resistance of 3500 ohms. So it's not hard to work out the value of shunts needed to adapt either of these meters to read higher currents. But what if you have a meter movement salvaged from a piece of equipment, and you don't know its resistance? Or even its current sensitivity? (It might already have an internal shunt, for example.) The simplest way is to measure it -- but don't just slap a multimeter across it, because that could damage the meter by passing too much current through it. The best way is to use the simple test circuit shown below.

Here the meter is connected to a 12V battery via a DMM (set to a suitable current range) and adjustable resistor RV1, which is carefully adjusted until the meter is just giving its full-scale reading. The DMM will then be showing the current needed to produce this current FSD -- in other words, the meter's sensitivity.

Now switch SW1 is closed, to connect the second adjustable resistor RV2 directly across the meter, as a shunt. RV2 is then adjusted until the meter reading is reduced to exactly half scale, while monitoring the total current via the DMM and, if necessary, adjusting RV1 to maintain it at the original FSD figure.

When the meter reading is exactly 50%, the value of RV2 will be equal to the meter's own resistance, because the current is being split equally through the two paths. So you can then disconnect the battery and use the DMM to measure the value of RV2 (by itself), to learn the meter's resistance.

Note that even if the meter should have a current shunt built inside it already, this test will still give you its effective sensitivity and resistance including any internal shunt. So the figures you get will allow you to work out the value of any new shunt you need to adapt it for measuring higher currents, regardless of what may be inside the meter case.

Simple Half-Wave Trapped Antennas Part 1 - The Basics

I was asked by a fellow ham to make a "how to" tutorial available on trap dipoles. A Power Point summary was suggested but I prefer to do a fuller, more complete explanation and, to that end, I am creating some posts on my blog. It is loosely aimed at the following objectives:

(1) Understanding the principles behind a trap dipole antenna without the concomitant EE degree math.

(2) Fabricating a trap antenna for the constructionally impaired from [mostly] ordinary "household" items.

I will probably break this up into four posts on the blog -- this one on the basics, a subsequent one on trap theory and calculations, a third on building traps, and a final one on putting the whole antenna together.

So let's charge right into it, shall we?

A trap [half-wave] dipole antenna is a device that will be resonant on two or more bands by means of one or more pairs of parallel resonant LC "tank circuits". Such a circuit is an inductance and a capacitance hooked together in parallel that exhibits the property of presenting an infinite -- or almost infinite impedance -- at its resonant frequencies.

But let's back up a minute and refresh our memories of dipole antennas. Sparing you the 1,000 words, here's a picture:

If you need a refresher on the derivation of the half-wave antenna formula, you can go here which I heartily recommend doing as it even has a video explanation of dipole antennas. Also, you will see that these antennas are fed with 72-ohm coaxial cable in the literature but, fret not, your 50-ohm coax will be fine for the moment. (But, down the road, you will come to realize that the RF loss of even the best coax is a mega-bummer if you like QRP.)

So, for the sake of argument, we would like to have a half-wave dipole for 40 meters. Hence:

Also, we wish to have a half-wave dipole for 20 meters. similarly:

And, while half-wave dipole antenna calculators abound on the internet, the above was derived from a site put up by West Mountain Radio who also went ahead and published the computations for all of the amateur HF bands, below.

So, armed with these, see that a 40 meter antenna is roughly twice as long as a 20 meter antenna and that we could probably use one antenna on both bands or actually terminate our coax into an antenna with BOTH 40m and 20m elements. And, while there is nothing wrong with either solution, there are drawbacks to each approach. (There are, in fact, problems with trap dipole antennas as well. Hey, nobody's perfect! )

Using the 20m band on both 20m and 40m will cause mismatch problems and compromise the efficiency of the 40m operation a great deal. Conversely, using a 40m antenna on 20m does not work out so well on 20m as it isn't a resonant match on that band. Still in all, it works far better than using the 20m antenna for 40m.

But I prattle.

The other alternative -- feeding a 20m and 40m dipole form one coax -- is called a "fan dipole" (for obvious reasons) and actually is better than a trap dipole but may have resonance or deployment problems. For example the multi-band fan dipole shown below is a stone bitch to deploy and tune without getting all of the bands set up so they don't "de-tune" adjacent bands.

In fact, I live in "CC&R hell" and cannot have the t*ri-band Yagi on the 70 foot tower we all lust after so a fan dipole in the attic had to do. Fortunately, I was able to lay it out so 40m, 30m, 20m, and 10m played nicely off of one coax but some re-arrangement was necessary. However, in the sense that one coax line and one run of wire can be deployed meets our needs most easily, then the question arises as to how this can be accomplished.

We could, of course, put up a run of 66 feet -- 33 feet on a side -- and put a switch in the middle of each side. When we wanted to work 20m, we'd just disconnect the extra 16 feet by opening the switch and Bob's your uncle, as the Brits say. Of course, this would become a monumental pain in the ass getting out the ladder or crawling up in the attic each time we wanted to change bands.

So what could we do? What if we had a magic device that would shut off the extra 16 feet when we worked 20m but did not when we worked 40m?

This is where traps come in.

It is a characteristic of a parallel LC circuit that, at resonance, impedance is almost infinite. Not to go into the hairy math in this discussion (it is discussed here) we can see that this is as good as the switch we wished to employ.

For example, if we configured our antenna as shown in the picture below, we would have the single run radiating element we desired and, when we transmitted on 20m the parallel resonant traps would offer an [almost] infinite impedance and the antenna would appear to be electrically resonant on 20m. Of course, the traps would not be resonant and the entire antenna length would be employed and, hence, resonant on 40m.

There's an added benefit to this as well. The inductor in the traps serve ot electrically add to the length of the antenna so therefore the overall length required for 40m is reduced by about 65 percent.

"Great", you say, "When do we start?"

..Well, that's for the next post on building traps.

Simple Half-Wave Trapped Antennas Part 2 - Calculating Trap Values

Antenna "traps" are nothing more than parallel LC (inductor/capacitor) circuits -- also called "tank" circuits. The derivation of "tank" and "trap" is intuitive once one understands how a parallel resonant circuit works or, more specifically, the nature of LC circuits.

There is a great discussion of these on Wikipedia and other renditions abound all over the internet.

But, basically, the take away on these is that -- at a circuit's resonant frequency -- the impedance of the series LC circuit is zero and the parallel LC circuit is [almost] infinite. Leaving the series LC circuit, the parallel resonant circuit's high impedance is due to the interaction between the capacitor[s] and inductor[s] in the circuit. I stole this image from the Wikipedia article but it shows this graphically:

Again, borrowing from the article, this animated diagram showing the operation of a tuned circuit (LC circuit). The capacitor C stores energy in its electric field E and the inductor L stores energy in its magnetic field B (green). The animation shows the circuit at progressive points in the oscillation. The oscillations are slowed down; in an actual tuned circuit the charge may oscillate back and forth thousands to billions of times per second.

Gradually without replenishment, this activity will dissipate. However, with an oscillating power source like energy from your transmitter or received at your antenna, the activity will continue uninterrupted by accepting the energy into the "tank" and or "trapping" it and not letting it pass further - hence the high impedance. (I am beginning to skate on thin ice here and suggest you seek hard tech info elsewhere before this erupts into a fount of formulas.)

The steps to build a trap are as follows:

 (1) Determine the resonant frequency of your trap.
 (2) Calculate the capacitance and inductance values.
 (3) Calculate the number of turns for your inductor.
 (4) Construct the trap.
 (5) Measure the resulting resonant frequency.

We'll cover steps #1 through #3 here and take up #4 and #5 in the subsequent post on building and measuring the traps.

For the moment, let's assume we're building a 40m/20m trap dipole. (For the sake of this whole series, we are talking about a two-band antenna. I will comment on constructing three or more band trap antennas in the building segment.)

Determine Resonant Frequency of the Trap
The resonant frequency of the trap should be a shade under the starting frequency of the higher frequency band you are designing, in this case 20m. So we are looking for a trap that is resonant in the range of slightly less than 14 MHz (around 13.5 MHz will be fine) so that it will cut off all frequencies above that frequency and acting like the automatic switch described in the first segment.

Calculate Capacitance and Inductance Values
This is almost arbitrary; you can obtain the same resonant frequency by selecting one LC combination and achieve the same resonant frequency with another difference set of values. Avoiding formulas, there's a nice parallel LC calculator to be found here -- and below, I have used it to demonstrate different values achieving almost the same resonant frequencies.

Note that with the selection of two pairs of components -- 27 pF cap and 5 uH inductor or 14 pF cap and 10 uH inductor -- we have achieved traps with approximately the same resonant frequencies and either one would be suitable. Of course, there are other design niceties to be observed in selecting values of components but that is left for your further study.

Further restrictions are, of course, the availability of the values selected. We can wind inductors (which will be covered below),  but capacitors come in discreet values. You can kluge a desired capacitance value by either connecting two or more in series where, like resistors, you are adding the reciprocal of the values) or parallel where you are just adding the values.

The voltage handling capacity of inductors and capacitors need to be considered as well. Since traps are at "the end" of the 20m antenna, they will be at a voltage node and that has to be accounted for. If you are working QRP any decent working voltage or gauge of wire should be fine; you can even construct the inductance (coils) from toroids. However, the kilowatt level is another matter so be very careful. (I will probably do a fourth segment on these considerations.)

Also, I need to pause here and enunciate a prejudice. Research into this subject will show you that hunks of coax can be used to build traps. As you can imagine, coaxial cable has, in effect, capacitance between the shield and center conductor and, as such, can serve both as an inductor or capacitor when wound around a form. The advantage to this that you only need to cut off the appropriate length and build the trap. I chose to keep the components separate so you got a better notion of the LC principles.

Calculate Number of Inductor Turns
Inductors take many forms: single-winding layer, multiple-winding layers, toroidal, etc. For our traps, we are constructing a single layer inductor; I pilfered these formulas from the 1981 ARRL Handbook:

Of course, you can enter this into your calculator laboriously, build a spreadsheet, or just use one of the internet sites to do this :

This covers the first three of our five above steps and, since this is somewhat long, we'll leave the rest for the third segment on actually building the traps, measuring the resonant frequency, and building the antenna.

Simple Half-Wave Trapped Antennas Part 3 - Constructing Traps

I was asked by a fellow ham to make a "how to" tutorial available on trap dipoles. A Power Point summary was suggested but I prefer to do a fuller, more complete explanation and, to that end, I am creating some posts on my blog. It is loosely aimed at the following objectives:

Constructing Traps
It goes without saying that almost anything can be used as a coil form for the inductor in a trap from air to iron. Classic traps in the days of yore was one made out of the Air Dux™ coil stock (shown below) where one sawed off a hunk for the correct inductance.

Of course, wood dowels, PVC, pill or other plastic bottles -- even card board tubes -- can be used. Take, for instance, this trap made out of a toilet paper tube, a hunk of wire, and a couple of 27 pF silver mica capacitors in series. Obviously, it will not stand up to a cat 5 hurricane - - or even a moderate drizzle -- but it can be "hardened" and weather-proofed and/or hidden in an attic where it will serve duty. (I Would not run more than 100 watts through it unless you have your track shoes on and your homeowner's insurance paid up.)

Of course, wood dowels, PVC, pill or other plastic bottles -- even card board tubes -- can be used. Take, for instance, this trap made out of a toilet paper tube, a hunk of wire, and a couple of 27 pF silver mica capacitors in series. Obviously, it will not stand up to a cat 5 hurricane - - or even a moderate drizzle -- but it can be "hardened" and weather-proofed and/or hidden in an attic where it will serve duty. (I Would not run more than 100 watts through it unless you have your track shoes on and your homeowner's insurance paid up.)

Also, there's no law that says you have to have a round coil either. A piece of wood with a square profile would work just fine. The take-away here is to use your imagination and see what you have on hand. My preferred material is PVC and generally in 1-inch to 2-inch diameters. The material is rugged, weather-proof, and easy to work with.

Here are a pair of traps I fashioned with some PVC, a 27 pF silver mica cap, some screws, and solder lugs:

The bottom line on constructing traps is the more perusal and experimentation you do, the better your antenna building kills will become. I did a post before this series with a bunch of pictures of traps I built for a vertical antenna You are invited to peruse those -- as the rest of the blog if you like.

Testing Traps
Once you build your traps you will need to test them; measure their resonant frequencies. I like to measure each component before I assemble the trap and then measure the resonant frequency of the resulting trap. As you will find out, components are not always dead on with respect to their marked value nor are the coils you wind going to exhibit the calculated inductance. Consequently, the resulting resonant frequency will not be what you calculate either. But not to worry. All you need to do is get things "close enough for government work".

First, let me digress and recommend you take a quick gander at this as a hint on tools you should be considering if you would like to progress further away from being an appliance operator.

All of that said about the "nice-to-own" tools, here's some notes and comments on ways to measure the resonant frequencies of your traps. First up is a video by yet another ham who does a great series on how-to projects, Peter Parker, VK3YE. It pretty much describes a way to use your radio and a broadband antenna (i.e., a long length of wire) to roughly determine the traps. It is fairly self-explanatory, illustrates a "cheap method" of doing this since you already have your HF radio, and it follows through with how Peter calculated the values necessary to achieve resonance at the desired frequency. Finally, he shows the resulting traps which -- along with my pictures and pictures of other traps on the internet -- should give you soem good ideas how to fabricate yours.

Unfortunately, he flashes by a piece of information from W8KI's website on traps that speaks to the desirability of component traps versus coaxial traps:

 - Trap loss greatly exaggerated by advertising hype
 - Traps should not be resonant at the actual planned operating frequency
 - Coaxial traps are more lossy than articles conclude
 - Coaxial stubs used as capacitors cannot be calculated using pF/foot unless the stub is a very small - fraction of a wavelength long
 - Coaxial stubs have a low Q (are relatively lossy) compared to normal lumped components

The actual website is here and it is definitely worth a glance as he develops this theme with calculations. The actual website is here and it is definitely worth a glance as he develops this theme with calculations. The bottom line is that you don't sacrifice anything by building traps with discreet components versus pieces of coax, the busy-body naggers notwithstanding.

In the link above where I made the recommendations as to the inexpensive test equipment you should avail yourselves of, I mentioned an RX noise bridge. These can be used in conjunction with the receiver on your HF rig to measure the resonant frequency and a good explanation on how to build one as well as use it can be found here. Note that the noise bridge will substitute for Peter's broadband antenna and provide you the antenna radiation resistance as well as the inductive or capacitive reactance.

There is a great article on an antenna noise bridge detector here.

..and another one here.

By this time you should have a couple of traps resonant just below the highest band of your dual-band antenna. (That is, around 13.5 MHz for a 20m/40m antenna.) Next we'll take up building the antenna itself. But rest assured, the toughest part of the job is done.

Testing the new tuner Too (two..whatevs)..

A review I did of the 4AQRP Antenna Tuner. I had at first bought the QRP Guys version and, while that was a perfectly serviceable unit, this kit is one that was a lot of fun to build and operate. The two features that were terrific was the PCB case and the red/green LED VSWR indicator. Other tuners are content witj one red LED but bright sunlight makes them hard to read. With a green LED brightening and an LED fading, you're a lot surer of where you you stand.

After assembling the 4 States antenna tuner kit, my overall impression was very favorable. Not 100 percent a fan of the kit aspect but about 98.7 percent. My only complaints are trivial: the pictures are somewhat ambiguous on some points and they trail the written instructions so you either pin up 3 sheets of pictures on your work area or leaf back and forth from instructions to the picture pages. Another complaint is that winding of the coil from the instructions is a tad confusing -- even including the pictures. And, thirdly, the polyvaricon caps received in the kit were festooned with connection tabs and it was not clear which tabs should be soldered where.

But these are truly petty gripes when offset by the final result. First, let me share what was learned.

One suspects the coil/switch assembly will be a toughie going in so it is well to prepare and study and prepare some more. Having a spare T106-2 on hand as well as a similar switch, I chose to wind a "practice" switch/inductor assembly for another tuner I am making. After a little struggle, it turned out pretty decently. However, the windings were the same for all twelve positions without the inductive "front loading" or "back loading" single-turn loops on the 4S tuner version. When I have some time, I shall ask why they do that.

Also, with respect to the polyvaricon leads/tabs, David, NM0S, points out that all but the three main tabs are not soldered to the board and can be cut off. In the hopes of clearing up a possible assembly ambiguity, here is the best picture from my build process that shows this.

That said, when I set out to test this, at first I had some difficulty getting it to tune my station antenna. It is a fan dipole for 40, 30, 20, 15, and 10 meters that somehow came together magically in the attic with decent SWRs on all of those bands. It's properties are that it presents a bitch of a time for the autotuners I possess on 17 and 12 meters. The internal K-2 tuner as well as the two LDG Z-11s on 17 or 12 meters have absolute fits! There have been a couple of manual MFJs and the QRPGuys tuner that were unable to obtain a match as well.

So that's the acid test. However, the 4State tuner handled it without breaking a sweat.

Once I got the hang of it, the unit was quite easy to obtain a decent match. The internal SWR bar graph on my FT-817 indicated that the radio was joyous over what it was seeing -- as did an inline Swan dual SWR meter. So, the little tuner will most likely accompany me across the way to the adjacent park and serve well with random long wires thrown up in the towering eucalyptus that abound there.

Anyway, here's some more shots of the testing process:

I just would like to add that it works wonderfully with my NorCal 40A radio. In the course of testing, I hooked up with a couple of folks using that rig and the tuner and got decent reports -- only to discover that I had left the VSWR bridge in line -- probably diminishing the output 990 mW a bit!

Closing the loop on the $5 AN/URM-25D

Closing the loop in the $5 AN/URM-25D, this beast cleaned up very nicely and is a good stand-in if the "A" unit fades. Love them both, to be honest. I had a teriffic WaveTek that was click-dial acurate but it started misbehaving inside and went back to WB6JDH from whom I seemingly "rent" my test equipment from. These opld fellas are easy to operate and my work is mostly HF with a little straying into the VHF regions..

..so life is good!

A while back, I walked picked up a couple of deals at the Pomona swap meet - - a Heathkit HG-18 sine and square wave generator that looked like it was fire-bombed and this AN/URM-25D signal generator that also looked like it was at death's door.

Actually, you don't "pick up" an AN/URN-25D at a swap. They're magnetic and..well, as I explained to the folks on the Antique Radio Forum:

Anyway, these things are not nearly as bad as they look. Military gear is virtually hermetically sealed and the innards are usually pristine. I used to collect military radio gear and some of the World War II and Korean War stuff -- particularly that which was to serve in tropical environments were water and fungus-proofed to beat the band. So, you can imagine how clean the insides of test equipment that dwelt in a service shop must have been babied was like.

Bottom line on this was that there was a cold solder joint on the frequency turret and, once repaired, the little beast was dead bang on. All that was left to do was get out the can of elbow grease and get 'er done.

..rock solid stable as well. Not bad for some old gal you pick up for $5 at a swap meet, eh?

Scoring and restoring old stuff: an AN/URM-25D

Another blast from the past, this is a pair of posts showing an AN/URM-25D I scored from a local swap meet for $5! It ruened out to be a real treasure as you aill see in the next post shoing the restoration.

At a swap meet some years ago, I scored an AN/URM-25D that looked like warmed-over dog poop outside but turned out to be very promising inside. I have another one of these that is now in the starting line-up when my Wave-Tek 3001 got a bad case of unlock-itis. So this one is warming up in the bull pen.

When I got it home and stuck it on the bench and checked it out just after the swap meet, it was dead on and rock stable. Aside from the cosmetics, the dial lights were burnt out and one of the bands was intermittent.

Here's the "before" pictures:

As figured when I forked over the five-spot to the seller, the outside looked tragic, but it did not look as though the inside had seen the light of day much and was relatively pristine.

I replaced the lights and related circuit with some bright LEDs but am less than happy with their brilliance - but they'll do. As for the intermittent band, I disassembled the unit and started poking and prodding but could not find anything amiss with the circuit. In the procees of doing that, however, I lifted the legs on a couple of components so they could be checked. When they were re-soldered, the band seemed to function correctly. So, I saw no reason to go further.

Here are the "during" pictures, The next post will be the results of sprucing it up..