Ham To Ham #26 - November 1997
73's Ham To Ham column
c/o Dave Miller, NZ9E
7462 Lawler Avenue
Niles, IL 60714-3108
USA
A bright spot in your day
Here's another workbench pin-up that you might want to keep somewhere handy, a simple LED
limiting resistor chart. Chart 1 shows the chart, which probably needs little further explanation.
Most light emitting diodes (LEDs) require a properly chosen current limiting resistor to keep
them from self-destructing, which can occur almost immediately if the resistor is inadvertently
omitted. The values in Chart 1, however, can act as a quick reference to the proper size of
resistor for supply voltages from 3 volts to 12 volts, using standard-resistance values and
keeping the LED forward current at about 10 to 12 milliamperes (which is safe in the majority of
devices). Some LEDs can handle more forward current (20 to 25 MA), in which case the value
shown can be cut in half, while others may perform better with less continuous current (5 or 6
MA), in which case the resistor value shown can be doubled. On average, however, most
currently made LEDs seem to work nicely with the exact values depicted in the chart at the
various supply voltage levels shown.
Approximate LED limiting resistor values
for 10 to 12 MA forward current
12 volts 1K ohms
9 volts 600 ohms
8 volts 470 ohms
6 volts 330 ohms
5 volts 220 ohms
3 volts 68 ohms
Chart 1
Chart 2 shows the results of a spot check of several LEDs from my own collection, with exactly
12 volts DC applied across the 1K resistor/LED combination:
LED color Forward current Voltage drop across LED
Yellow 9.7 MA 1.98 volts
Green 9.57 MA 2.09 volts
Red 9.30 MA 2.36 volts
Amber 9.72 MA 1.93 volts
Blue 8.77 MA 2.91 volts
The limiting resistor was kept constant at 1K ohms.
Chart 2
From my brief experiments, the red LEDs seem to have the greatest variation in forward current
through, and voltage drop across, the device from one to another, in a random pick ... just an
interesting observation. NZ9E
Joys of the J-Pole
Here are a couple of interesting add-ons to the classical J-Pole antenna from M. Marcel
Chapleau VE2GMZ of Quebec Canada: "A few months ago, I decided to build my own J-Pole antenna for 2-meters and 70CM, instead of buying a considerably more costly factory-made unit. In going through my old ham files, I ran across a design that looked about right for my needs. The original was written-up by John Post KE7AX and I copied his design for my first attempt.
The antenna described by KE7AX worked out well, but I did notice two things that I felt could be improved upon. On a J-Pole antenna, the coax cable is attached to the lower portion of each element (about 2-1/4" up from the horizontal connecting piece) while watching the SWR presented to the transmission line. At best, I could only achieve about 1.3:1 on 2-Meters and 1.6:1 on 70CM. Not bad, but room for improvement.
Squeezing the 19" vertical radiating element closer to the 60-3/4" element (at the top of the 19" pipe), showed a drop in SWR to very close to unity. Based on this finding, I made up an 'L' shaped angle bracket 2" long by 1" wide and with a 1" drop-leg, and configured a 1-1/2" slot across the top 2" length. This allowed me to attach the 'L' section to the flat top of the 19" radiator's cap-piece with a single machine screw, and provided me with a 'Fine SWR Adjustment'. Now I can simply slide the 'L' piece back and forth on the 19" element until the SWR is as close to 1:1 as possible.
With just that addition, I installed the dual-band J-Pole at my home QTH and used it quite often for several months. I was able to reliably access a repeater 50 miles away, as well as the Russian MIR space station on a number of occasions. I felt, however, that the angle of radiation might be too high for good space communications (MIR was mainly only usable between 20 degrees to 65 degrees), so I decided to try something else. I constructed two sets of 4-spoked radials, one for 2-meters and one for 70CM. The 70CM radials (each 6-1/2" long) were then positioned 6-1/2" down from the bottom of the 'J' cross-over piece, and the 2-Meter radials (each 20" long), another 13-1/2" further down (a total of 20" from the 'J' cross-over piece). The details of the entire antenna are shown in Figure 1. My reliable repeater 'reach' now increased to 95 miles away, and I'm able to talk with a friend 45 miles down-range on simplex, just using the J-Pole with it's added radial 'skirts'. Communications with MIR were not as gratifying, however, and I suspect that perhaps the angle of radiation may now be too.
Needless to say, for terrestrial coverage, I've been very pleased with the results for my meager investment, and I thought my experiences may have appeal to others in the ham community."
Moderators note: Marcel's dual-band J-Pole dimensions are shown in Figure 1, and the radial mounting ring that he made up is pictured in Photo 1. Normally, a J-Pole doesn't need a ground plane or radials to perform correctly. I suspect that Marcel's added radial system (for both 2-Meters and 70CM) tends to "decouple" the coaxial transmission line from the antenna, so that any common-mode currents that might exist on the transmission line are suppressed, and the shield of the transmission line is no longer a part of the active "radiating" portion of the antenna. This isn't uncommon at VHF and UHF frequencies, proving that "apparent" theory and "real-life" practice are often somewhat at odds. Never argue with success!
Marcel has kindly offered to make up copies of the special radial mounting ring that he devised for his antenna for Ham To Ham column readers (see Photo 1). The sample radial mounting ring that he sent me is nicely machined from 3/4" aluminum stock and has four holes around it's perimeter to accept copper welding rod 'radials'. The radials are then locked into place with Allen-head setscrews at all four points. A 1/2" hole passes through the center of the ring for fitting onto the 1/2" O.D. copper pipe used in constructing the antenna, and it too is locked into place with an Allen-head set-screw. Marcel is offering a retrofit radial kit consisting of two mounting rings and two sets of radial wires, for those who might not be able to do the machining themselves, for $20 (US funds) including shipping. Write to VE2GMZ at the address at the end of the column for further information or to order the kit just described.
A switch for a switch
Craig Stimson VA3DCS offers this suggestion for an easy way to build a packet/voice TNC/Mic switch: "Do you have a 25-pin computer A-B data transfer switch just gathering dust on the shelf? Looking for an inexpensive way to switch between my packet TNC and my microphone, I decided to give the spare data switch that I had a new purpose in life! Something of a role-switch for a switch! I fabricated 3 special cables: one DB-25 to an 8-pin mic connector that would plug into my transceiver's mic input jack, one DB-25 to the audio connector used on my TNC, and one DB-25 to the correct gender 8-pin connector into which my transceiver's microphone would mate. Connecting everything together as shown in Figure 2 gave me the mic to TNC A-B switch that I needed, for just a fraction of the cost of a commercially made unit. Just be sure to be consistent in your wiring so that all pin-to-pin interconnections are correct for your particular setup, and that shielded pre-made 'DB' connector cables are used for the external adapters."
Moderator's note: That's a clever "alternate" use for an A-B data switch Craig. Even if you don't happen to have one on the shelf, they can generally be found quite reasonably at every ham and computer fest. Also check out the catalogs of some of 73's advertisers; you can sometimes find the DB-25 A-B switches for about $5 and pre-made dual-ended DB-25 cables for $2 or $3. In addition to using only shielded interconnecting cables, it's also a good idea to ground all unused conductors at both ends; this will help in fending off RFI induction into the low-level microphone circuit.
Battery BASICs
Joseph Gabus AB5RE offers this basic program to readers: "Back in the February 1997
issue of 73 Amateur Radio Today Magazine, J. Frank Brumbaugh W4LJD wrote a nice
article entitled The Gel Cell Storage Battery (A great little power supply), in which he
makes the case for why it may make better sense at times to use lead-acid based gel cells
to power portable amateur gear than other competiting battery technologies. To most
effectively use the information in Frank's article, however, it's necessary to compute the
expected operating time based upon the expected average transmit/receive duty cycle vs.
the battery's ampere-hour capacity for each different battery type (ie., amp-hour capacity)
under consideration. To make the job easier and less prone to computational error, I've
come up with a simple BASIC program that can be used to complete the task. Since it's
written in BASIC, it's easy to modify for those familiar with that programming language,
and it will run under on any computer that can utilize a BASIC interpreter; if you're using
a modern Windows (reg trade mark) computer, you can use the QBASIC program in
DOS.
The actual program BASIC instruction lines are reproduced below, just type them in, and
save them exactly as shown, via your BASIC interpreter:
=====================================================================
10 CLS
20 PRINT : PRINT "NAME OF RADIO: "; N$; " LAST COMPUTATION"; X; "HOURS WITH A"; Ah; "AMPERE BATTERY."
30 PRINT : PRINT : PRINT SPC(25); "GELL CELL COMPUTATION PROGRAM ": PRINT SPC(5); " TO DETERMINE OPERATING TIME WITH YOUR RIG, BEFORE RECHARGE IS REQUIRED."
40 PRINT SPC(23); "VER. 1.1 By: Joseph T. Gabus, AB5RE": PRINT
50 PRINT "TEST DATA: 12 Ah BATTERY, 1.5 AMP TX, 0.2 AMP RX = 40 HOURS OPERATING TIME."
60 PRINT: INPUT "NAME OF RADIO ", N$
70 PRINT : INPUT "AMPERE HOUR GELL CELL BATTERY CAPACITY ", Ah
80 AHR = .05 * Ah: PRINT "CHARGE / DISCHARGE RATE FOR 20 HOURS = "; AHR; " AMPERES/HOUR"
90 PRINT : INPUT "TRANSMITTING AMPERES = ", TA: INPUT "RECEIVE AMPERES = ", RA
95 REM Note that the formula below assumes 50 minutes of RX time and 10 minutes of TX time. Contest (or other high ratio transmit operations) will require a change to be made to the formula.
100 AC = (5/6*RA)+(1/6*TA)/2 : PRINT "AVERAGE CURRENT DRAIN PER HOUR = "; AC
105 REM The next 3 lines allow for the use of the accessory(s) of your choice.
110 REM PRINT "ACCESSORY: DSP-40 CURRENT DRAIN = 1.0 AMPERES
115 INPUT "ACCESSORY CURRENT DEMAND IN AMPERES (Default is zero) = ", ACD
120 AC=AC+ACD: PRINT AC; " = AMPERES WITH ACCESSORY."
130 X = Ah/AC
140 PRINT : PRINT SPC(20); "APPROXIMATE OPERATING TIME = "; X; " HOURS."
150 PRINT "HARDCOPY PRINT OUT? <Y/N> "; : INPUT C$
160 IF C$="Y" OR C$="y" THEN LPRINT "RADIO "; B$, C; "AMPERE BATTERY", INT(Y-1); " HOURS."
170 PRINT "QUIT PROGRAM NOW <Y/N> "; : INPUT A$
180 IF A$="Y" OR A$="y" THEN GOTO 210
190 GOTO 10
200 REM DATA SOURCE: "The Gel Cell Storage Battery" - Frank Brumbaugh, W4LJD, 73 Magazine, Pages 41-42, February, 1997.
210 END
220 REM Note that your experimental results may differ somewhat. If your radio quits at 72 hours, for instance, and the computed answer was 80 hours of projected operating time, then change Line 130 to X=(Ah/AC)-8. This adds a bias to the program to compensate for the actual parameters of your particular Gel Cell battery. Batteries will change Ah capacity with age, temperature and percentage of charge (time since last recharge).
=====================================================================
I've called the program shown above GELCELL.BAS. After inputting the data and calculating the results, the program gives you the opportunity to print a hard copy of the results as well as to compute another set of variables for a different battery, simply answer "y" or "n" to these questions. There is also reminder printout at the top of the screen showing the results for the last computation. Line 115 gives the user the option of adding any other constant-current accessories into the calculations. It can be removed if none are ever used or simply push
< ENTER > to default to zero.
By using GELCELL.BAS, I found that my MFJ-40 CW transceiver (which draws 1 amp during
transmit and .05 amp during receive) could potentially operate for 80 hours, before
recharging of the battery would be needed during an emergency, using just a 10 ampere-
hour gel cell battery. Adding the constant-current of a Radio Shack (reg trade mark)
DSP-40 signal processor however, raised the current demand enough to require
approximately a 90 ampere-hour battery for roughly the same operating time. In that
case, better bring along a husky assistant to help carry the battery!"
Moderator's notes: I asked Frank Brumbaugh (W4LJD) to comment on Joe's implimentation of the information outlined contained in his article, "The Gel Cell Storage Battery", Frank wrote back:
"When calculating transmitter current drain, the duty cycle (the amount of time that the
transmitter is actually drawing full current during the transmit mode) must also be taken
into account, as well as the ratio of receive (or tuning around time) vs. actual on-air QSO time. A
50% duty cycle is generally assumed for both CW and SSB to be on the safe side. Other
modes, such as RTTY, FM or AM, must be figured as 100% duty cycle while transmitting. Let's further assume that a normal routine might be tuning or listening for 50 (5 / 6) minutes out of an hour's time, while transmitting may only occupy 10 minutes (1 / 6) of that same hour. With the 50% duty cycle of CW or SSB, the following formula can be applied:
Current during receive = .050 Amperes (50 milliamperes)
Current during transmit = 1.0 Ampere (1,000 milliamperes)
Battery's rated capacity = 10 AH (Ampere hours)
(5 / 6) of .050 + (1 / 6) / 2 = Amperes for one hour
.042 + .083 = .1253 Amperes (125.3 milliamperes) for one hour
Battery AH divided by Amperes-per-hour = maximum operating time
10 over .1253 = 79.8 maximum estimated hours of operating time
The above assumes that the qualifications mentioned previously are completely accurate
of course. Any estimate of actual duty cycle is simply that ... an estimate. Additionally, one would not want to run their battery right down to fully discharged, at least not intentionally.
Lacking a computer, you can simply use a hand-held calculator and get the same results by using the formula shown above.
By the way, anyone who might be interested in pursuing the question of the care and feeding of Gel Cells more thoroughly can contact Power-Sonic Corporation at P.O. Box 5242, 3106
Spring Street, Redwood City, CA 94063 (Tel: 415-364-5001, FAX 415-366-3662) and
ask for a copy of their free technical manual covering their line of Gel Cell batteries. This is a very well written treatise covering all aspects of Gel Cell technology. Power-Sonic is a friendly company and they courteously provided me with a great deal of assistance while I was doing research for my 73 Magazine article. Frank Brumbaugh (KB4ZGC)."
Moderators note: Joe modified his program slightly to reflect the factors Frank mentioned, that is: 1/6 RX vs. 5/6 TX duty cycle. If you assume your TX to RX duty cycle to be very much different from these figures, then you'll want to change Line 100 in the program as mentioned in the REM in Line 95.
Errata
In addition to being the least expensive of the various battery technologies to manufacture, the SLA (Sealed Lead-Acid) Gel Cell packs, written about by Joe Gabus and Frank Brumbaugh, are also the easiest to determine how much relative charge is left in them. The reason for this is that the terminal voltage of lead-acid cells drops at a linear and predictable rate, from fully charged to fully discharged, allowing us to simply measure the voltage across the battery to come up with a reasonably accurate estimate of the amount of usable energy left in the battery. When fully charged, a 12 volt SLA battery pack will read an open terminal voltage of 13.08 volts (2.18 volts per cell). At 50% charge, the 12 volt pack will read 12.54 volts (2.09 volts per cell), and at 10% charge, a 12 volt pack reads 11.88 volts (1.98 volts per cell). When charging, the terminal voltage of a 12 volt SLA pack will rise to between 13.8 and 14.4 volts, depending upon the actual output voltage of the charger (SLA battery packs are best charged with a constant voltage as opposed to a constant current as is the case with NiCd and NIMH cells). As mentioned, this linear terminal voltage drop-off in lead-acid cells (again, unlike NiCd and NIMH cells), makes it quite practical to use an expanded-scale DC voltmeter to measure the relative percentage of charge left in the pack. This means that an easy-to-build meter, like the one described by Frank Brumbaugh in last month's Ham To Ham column, can be really helpful in preventing excessive drain-down (see 73's Ham To Ham column for October 1997). That's all for this month, be sure be with us again next month for more worthwhile tips, ideas and suggestions ... ham to ham!
Murphy's Corollary: The test-lead on any multimeter will break just before you've finally
"zeroed-in" on the fault that you're troubleshooting. Having to stop and fix the test lead is Murphy's contribution to building your patience and character!
As always, many thanks to those readers who've contributed their time and ideas to this month's column, including:
M. Marcel Chapleau VE2GMZ
4, De Langloiserie
Blainville, QC J7C 4L6
Canada
or
E-Mail: ve2gmz@sympatico.ca
Note: The 2-Meter/70CM radial kit mentioned in the text is available from VE2GMZ
Craig Stimson VA3DCS
302-2311 Ontario Street
Oakville, ON L6L 1A5
Canada
Joseph T. Gabus, AB5RE
Route 2, Box 91-A
Bowie, TX 76230
J. Frank Brumbaugh W4LJD
P.O. Box 30
c/o Defendini
Salinas, PR 00751
If you're missing any past columns, you can probably find them at 73's Ham To Ham column home page (with special thanks to Mark Bohnhoff WB9UOM), on the world wide web, at: http://www.rrsta.com/hth
Note: The ideas and suggestions contributed to this column by its readers have not necessarily
been tested by the column's moderator nor by the staff of 73 Magazine, and thus no guarantee of
operational success is implied. Always use your own best judgment before modifying any
electronic item from the original equipment manufacturer's specifications. No responsibility is
implied by the moderator or 73 Magazine for any equipment damage or malfunction resulting
from information supplied in this column.
Please send any ideas that you would like to see included in this column to 73 Magazine's Ham
To Ham column, c/o Dave Miller NZ9E, 7462 Lawler Avenue, Niles, IL 60714-3108, USA. We will make
every attempt to respond to all legitimate ideas in a timely manner, but please send any specific
questions, on any particular tip, to the originator of the idea, not to this column's moderator nor
to 73 Magazine.