Ham To Ham #23 - August 1997

73's Ham To Ham column

c/o Dave Miller, NZ9E

7462 Lawler Avenue

Niles, IL 60714-3108

USA

E-mail: dmiller14@juno.com

You're invited to send me your tips, suggestions, ideas and shortcuts, as always, to the

addresses shown above. I'm always in the market for new and interesting contributions like this

first item from Ken Guge K9KPM. Aside from being an interesting story in and of itself, it

illustrates how Ken used logical troubleshooting techniques to solve a seemingly illogical

problem.

The mysterious capacitor

"I recently encountered what a first seemed a strange problem with my broad-band VHF/UHF

discone antenna. One of the beauties of the discone design is its very wide-band frequency

capabilities; this one (a commercially made unit) is specified as usable on transmit from 2

meters, on up through the 23 centimeter ham bandÖand an even grater range of potential

usability as a receive-only antenna.

Over the winter, however, my discone's SWR rose dramatically (to over 3:1 at 2-Meters). It

reached the point where the antenna became virtually unusable for either receiving or

transmitting. I first checked the indoor end of the feedline with an ohmmeter, and found that

there was some measurable shunt resistance; this in an antenna that should have displayed a

completely open circuit to DC. Going a little further, I also found that it was acting very much like

a fairly high value capacitor ... about 3,000pfd. in fact! At 2-Meters, that 3,000pfd. (.003ufd.)

would be enough to effectively by-pass all but the strongest of 2-Meter signals to ground,

working out to about an ohm of capacitive reactance at 144MHz.

My first thought was that perhaps water had somehow worked its way into the coaxial (RG-8X)

cable feeding the antenna, but disconnecting the coax at the far end, and repeating the

measurements, proved conclusively that the coax itself was perfectly all right ... the problem had

to be in the antenna. But what could go wrong with a nicely-machined, well-sealed antenna's

main connection block? There was virtually no open gap for water to enter the connecting block

from the sides nor from above, but yet disassembling the entire machined piece, showed a

considerable amount of internal corrosion and even a small amount of liquid water itself. Figure

1 shows a side view of the basic construction of the antenna Ö not particularly complicated nor

subject to this type of problem. What might possibly have caused this seemingly inexplicable

condition to develop? The antenna's coaxial connector is facing downward, it was unlikely that

water might have worked its way in via this route. Ditto for the rest of the discone's construction.

There just wasn't any obvious way for moisture to enter in a liquid form ... only via humidity in the

air. Here's what I feel happened.

The interior or the connection block has a small, hollow cavity, which though not penetrable by

rain, is subjected to infiltration by moist, humid air (not being hermetically sealed). We have our

fair share of moist, humid days in the Chicago area, often followed by quickly changing

temperatures that can drop amazingly rapidly (sometimes 60 or 70 degrees in just a few hours).

If moist air gets into the connector block's cavity, and then is subjected to a dramatic cool-down,

the moisture in the trapped air will condense out in the form of liquid water. Several episodes of

such a natural occurrence can cause a significant buildup of water inside of a tight (but not

perfectly air-tight) container. I feel that the discone's connecting block cavity was such a

container. Over time, of course, the water worked on the surrounding metal pieces, causing

severe corrosion and changing the pure water into something more like an electrolyte. That

electrolyte, and the otherwise insulated metal pieces inside the block, resulted in a .003ufd.

electrolytic capacitor being developed right inside of my VHF/UHF antenna! Isn't nature

wonderful!

Now here's the moral. If you live in a climate such as that found near Chicago, it might be best

to consider either sealing relatively air-tight connections entirely, or if that's not practical, then

leaving enough free-air circulation space so that air exchange can take place more rapidly when

climatic conditions shift quickly. In that way, humid air has a chance to be flushed out before it

can condense and deposit it's water into the open cavity. In the case of my discone antenna, I

chose to fill that open cavity above the bottom connector with Stuf (reg. trade mark). That's what

it's called ... Stuf. It's a Teflon (reg. trade mark)-based grease that's compatible with electrical

connections, and is made by Cross Devices of Cutchogue, NY. It's described as a dielectric

waterproofing filler for coax connectors. I bought my tube of Stuf from AES in Milwaukee, but it's

probably available elsewhere as well. I packed the grease into the cavity of the connection block

so that some of it actually oozed out when everything was reassembled and a new connector

was installed, assuring me that the cavity would now be virtually air-tight (at least tight enough to

prevent a similar occurrence in the future). We'll see.

After reassembly, the antenna showed perfect continuity through the connection points and no

shunt impedance whatsoever, so I'm reasonably confident that the job will hold. I hope that my

experience may have some value to other Ham To Ham readers who've encountered similar

problems with outdoor connections. Remember to stick with it, there's an explanation for

everything!"

Moderator's note: Thanks for letting us in on that experience, Ken. In the future, I for one, will

view my outdoor antenna connections very differently! Ken is absolutely right when you stop to

think about it. Any enclosure that's capable of trapping moist, humid air is a potential water

reservoir, just waiting to happen, when a quick change in temperature takes place. Filling that

reservoir with an RF compatible grease (such as he did), is probably the best insurance against

an event like K9KPM experienced. Good tip Ken!

Circular memory

Here's a clever, easy-to-implement tip from Frank Brumbaugh W4LJD that won't void a single

manufacturer's warranty: "If you ever have difficulty remembering what band your antenna tuner

might actually be tuned to, then here's the gadget for you! It's never a good idea to key your

transceiver into an unknown load, as can easily happen if you've forgotten what band (or band

segment) you last used your tuner (antenna transmission line matching network) on. Many

tuners are not marked with regard to actual band or frequency range, so it's often difficult to see

where the tuner is set just by glancing at it...but no more!

Take a look at Figure 2 and make up something similar for yourself, either out of cardboard or

artist's drawing board. What's shown in Figure 2 is somewhat reminiscent of the old circular

slide rules used by engineering students before the advent of electronic calculators and now

palmtop computers. It's just a circular cardboard cutout that mounts onto another piece of

square or rectangularly shaped cardboard, with a single screw through its center and a nut

placed on from behind. All of your normal ham band haunts can be shown around the perimeter

of the movable circle, and when you've tuned everything up correctly, move the cardboard circle

to the proper position to indicate what band or band-segment you're tuned to. That's it, nothing

elaborate, just an always-close-at-hand reminder for those of us who need it...don't laugh, if you

don't need it now, you will!"

Reach for the stars

Here's a tip from Thomas Hart AD1B for making it easier to operate through the RS-12 satellite:

"Since becoming 'seriously' involved in hamming through the RS-12 satellite, I've been seeking

out ways to make the experience even more fun. Using the satellite involves transmitting on the

low end of the 15-meter phone band and listening on the satellite portion of the 10-meter band.

Using only a Kenwood TS-430S, I've been able to put the transceiver's built-in split-VFO to work

as the means of generating the 15-meter up/10-meter down frequency offset needed. As time

went on, I thought that it might be interesting to hear my downlink signal in real-time too, so I

tried using another 10-meter-only transceiver that I owned for that purpose. The results were

disappointing, perhaps partly due to the transceiver itself not being sensitive enough, perhaps

partly due to desensing caused by my own 15-meter transmit signal. I heard myself on the

downlink, but it wasn't nearly what I had hoped for in terms of signal strength and reliability.

My next step was to try listening for myself on RS-12's 2-meter downlink instead. I purchased a

reasonably priced 2-meter to 10-meter converter from Hamtronics (Tel: 1-716-392-9430), used

the 10-meter-only transceiver as the tunable IF for the converter, and ended up very satisfied

with the overall results. The use of a sensitive receiving converter ahead of a mediocre 10-meter

transceiver seems to be an economical answer for a better downlink monitor. Since the noise

level and sensitivity are determined in the 2-meter converter, even a converted CB SSB

transceiver may work well enough as the IF, demodulator and audio stages of a setup like this.

Why not give it a try if you happen to be strapped for cash, but would still like a few bells and

whistles in your satellite station?"

Cable shrinker

Here's another good mobiling suggestion from Phil Salas AD5X: "In my mobile VHF ham

installation, I like to use RG-8X (or RG-8M from Radio Shack (reg trade mark)) to bring the RF

from radio at the dashboard, out to the hatchback antenna mount in the rear. To then exit the

vehicle, I've rigged up a short (1 foot) section of RG-174/U to get through the car's hatchback

gasket, and then to the outside antenna mount.

RG-174/U 50 ohm coax is the perfect size to use...being just a tenth of an inch in diameter...it

easily feeds through my car's hatchback weather-stripping without spoiling the purpose behind

the weather-stripping gasket. Splicing the RG-8X to the RG-174U takes place via a pair of BNC

connectors and a BNC feed-thru barrel. Rigging a BNC connector for the RG-174/U can be a bit

tricky, but I've had good results using a crimp-on type of BNC plug (intended for RG-58 coax) but

soldering it instead to the miniature RG-174/U coax.

Here's how I've done it: First, I tin the lip of the BNC's collar, then strip off about 1/2" of the RG-

174/U's outer black jacket. I unbraid the exposed shield and fold it back along the remaining

jacket. Next, I'll strip off all but 1/8" of the center conductor's insulation, cut the center conductor

to 1/4" length, tin it, and slip the BNC center pin over the tinned end, carefully soldering it in

place with minimum heat and very little additional solder. If too much solder is used, the center

conductor pin may not fit back into the BNC's shell properly. If done correctly, you should now

be able to insert the RG-174 finished cable and pin into the BNC's shell far enough so that the

center pin is seated firmly and the tip protruding just shy of the end of the BNC shell. The final

step is to solder the RG-174/U's braid to the previously tinned collar of the BNC plug assembly,

again using the absolute minimum amount of heat to do the job. Allow plenty of cooling time and

don't move the assembly during the cool-down period...these tiny cables can't withstand much

heat, so it's best to practice on a scrap piece first.

Last, but not least, I put a short length of heat shrink tubing over the BNC's collar and part

of the cable, and shrink it in place. The addition of the shrink tubing gives a protective,

professional looking touch to the job. Finally, connect the open end of the RG-174/U to the

antenna mount that you're using, again finishing up with a short piece of heat-shrink tubing for

stress resistance and weather tightness. The entire cable assembly should hold up well if

reasonable care is taken to make sure that it doesn't get "pinched" at any time in the hatchback-

door to frame interface. It's also one that you'll be proud to show off to your ham buddies."

Battery tap tip

This idea is from Jerry Lagersbrom AA0MO: "Here's a tip to keep in mind if you need to make a

high current connection directly to your automobile's 12 volt battery, such as for that new mobile

ham rig you just bought. Instead of splicing into the existing battery connections at or near the

battery's terminals, simply use the "spare" set of terminals that already exist on many of the

current line of auto batteries. Since some automobile makers use the side-mounted terminals for

their main connections, and some use the top terminal scheme, battery makers will often provide

two sets of terminals on new and replacement batteries. It helps to keep down the needed

inventory that dealers have to keep on hand. It also makes it very easy for the ham radio mobile

operator who wants to tap into the battery right at the source itself...which is the recommended

way of doing the job. Simply purchase the correct terminal kit (either top or side) opposite from

what you car's manufacturer has chosen, and use those terminals for your mobile rig. When it

comes time to sell the car, it's an easy matter to remove the radio's cabling, since you didn't have

to splice into the original factory wiring for your installation. It also makes it easy to disconnect

your added wiring should it become necessary to isolate a future power drain problem...at least

you can easily exonerate your ham gear from the formula."

Murphy's Corollary: Any rechargeable tool battery will run-down just moments before the

last of the important tasks is completed.

Many thanks to the contributors who make this column worthwhile each month, without their

continued input, I'd run-down pretty quickly. They include:

Ken Guge K9KPM

1107 E. Woodrow Avenue

Lombard, IL 60148-3127

e-mail: k9kpm@juno.com

J. Frank Brumbbaugh W4LJB

P.O. Box 30

c/o Defendini

Salinas, PR 00751

Thomas Hart AD1B

54 Hermaine Avenue

Dedham, MA 02026

Phil Salas AD5X

1517 Creekside Drive

Richardson, TX 75081

Jerry Lagersbrom AA0MO

1301 1st. Avenue

Plattsmouth, NE 68048

Be sure to check out the Ham To Ham column's home page 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 all correspondence relating to this column to 73 Magazine's Ham To Ham column,

c/o Dave Miller, NZ9E, 7462 Lawler Avenue, Niles, IL 60714-3108, USA. All contributions used

in this column will be reimbursed by a contributor's fee of $10, which includes its exclusive use

by 73 Magazine. We will attempt to respond to all legitimate contributor's ideas in a timely

manner, but be sure to send all specific questions on any particular tip to the originator of

the idea, not to this column's moderator nor to 73 Magazine.

Note: If you would like a copy of any of the circuit diagrams or figures referred to in

this column, simply send a SASE to Ham To Ham Column, c/o Dave Miller NZ9E,

7462 Lawler Avenue, Niles, IL 60714-3108 and indicate the month and circuit or

figure name on your request. No requests will be honored without a self-addressed and

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