Articles by N1GY

HOMEBREWING A DESK MIC By Geoff Haines, N1GY

This project all started with the desire to have a desk mic to use with

my Yaesu 747GX HF transceiver. A quick look in the catalogs for several

purveyors of ham radio gear, and an even quicker look in my wallet

convinced me to think about some kind of alternative. The desk mic went

on the back burner.

Then one day, my wife asked me to remove the handle from an old frying

pan that had seen better days. The non-stick coating was toast, so she

wanted to use the pan (without the handle) under a flowerpot. The task

was done and I was about to toss the handle when inspiration struck.

Well, it was either inspiration or indigestion, but I got an idea.

I had a hand mic lying around the radio room with a 1000 ohm electret

element and a good coil cord. A new PTT switch was not going to empty

the wallet, and I already had purchased the 8-pin plug at the last

hamfest. The pot handle, when held in the vertical position, had some

of the appearance of one of the fancier desk mic's that are out

there.... A plan was already forming.

The first order of business was to create a base

for the pot handle/desk mic. Since I am also an amateur

woodworker, a simple frame of pine (seeDiagram 1) was

easy to build from scrap. It's shape was dictated by the

need to tip the pot handle to the right angle for a desk mic.

The top and bottom of the base were made from plastic sheet that I

have been dragging around for the last several years. The source was

long since forgotten. Thin plywood, sheet aluminum, or any material

that can be drilled for the switches can be used if you don’t have the

same material as I did.

The top and bottom pieces were secured to the base

with screws and Radio Shack rubber feet were glued to the bottom.

The pot handle was secured through a rectangular hole in the top with

hot glue. A small piece of black fabric (thanks to my wife, She said it

was black bias tape, whatever that is) was hotglued inside the hanger

opening in the end of the pot handle. With the non-electronic

components out of the way, it was time to turn to the wire and

solder side of the project.

The electret mic element wires to the PTT switch were

extended with about 4" of hookup wire. The coil cord was placed in

a notch at the back of the base so it would be secured when the

bottom panel was screwed into place. So far so good. Then it

hit me; I have to power this mic somehow! The Yaesu uses a

dynamic mic normally and has no provision for an electret type mic.

OOPS!

By the sophisticated technique of getting on the local

repeater (thanks Big Stick) and asking around (HELP!!!) I was

able to find a circuit to power the mic with a 9-volt battery.

Many thanks to one and all who helped this "appliance operator"

with his homebrew project.

The circuit I used is very simple. It consists of the 9-volt

battery with the positive side running through a 1k ohm resistor

to the "hot" side of the mic and the neg. side of the battery to the

ground side of the mic. A capacitor then connects the hot side

of the mic to the PTT switch and from there to the appropriate

pin on the mic plug.

I inserted a SPST switch in the Positive line from the

battery to allow for shutting the mic down when the radio was

not in use. The PTT circuit runs through the other side of the

PTT switch. A DPST Momentary switch was used instead

of the stock hand mic PTT because it was better suited to

mounting on a desk mic base.

I did not bother with switches or wiring for the various

UP, DN, or Fast/Slow switches that the stock Yaesu hand mic

has because I don’t use them anyway.

With the mic completed it was time for the dreaded

ON-AIR test. First reports were of the "good news/bad news"

sort. Yes, the mic worked, PTT switch turned thetransmitter on

and off as it was supposed to, the element modulated, BUT....

I was told, it sounds very "sharp". That is, no lows, great highs,

some mid range. Maybe a good DX mic, but you wouldn't want

to listen to it all day long. HMMMMMM? back to the drawing

board.

I compared all the various circuits that I had found in my

search on the repeater and the net and the phone etc. I discovered

that there were many different values indicated for the resistor and the

capacitor even though the basic circuit was the same. No one I talked

to was able to tell me what values to use to improve the audio quality,

so I resorted to the old cut and try method.

Success came on the first try, I think! I changed the Capacitor

from 10 microfarads to .1 microfarads, and tested again. Reports

indicated good intelligibility, still high, but better than before. I have

a feeling that the proper values will vary with the specific radio

and/or element. I plan to do more experimentation with other values

for both the resistor and the capacitor in the future to see if I can

tailor the audio even better. Maybe even add another switch to

be able to go from a setting best for DX to a setting better suited

to rag-chewing etc. For now however, the mic works and I built it

myself! WOW!

I learned many things from this project.

1. Even an "appliance operator" can homebrew gear.

Just take it slow,don’t overwhelm yourself trying to build a better all

mode, all frequency tranceiver as your first project.

2. Read and research your project thoroughly

before you spend money on components etc.

3. When all else fails, cut and try still works. Be

careful though, don’t do cut and try with a Mega-buck radio.

4.The satisfaction of "I built it myself" is worth more

than that 300 dollar desk mic.

In conclusion, let me say that I had lots of help from many

other hams in designing and building the mic. Some I knew from the

local clubs and on air, but also from hams as far away as New

Jersey, Ohio, and Jamaica whom I had never met before. One and

all, they came to my aid with suggestions, signal and audio reports,

and most of all, encouragement to stick with it. Thank you all.

A PORTABLE MASTING SYSTEM

FOR EMERGENCY USE

By Geoff Haines, N1GY

The impetus for this project was an A. R. E. S. deployment in support of a local rescue squad for a training exercise. A fellow A.R.E.S. member, who owns a pick-up truck, utilized a 15 foot piece of antenna mast as a field expedient tower to increase the coverage area of his mobile radio.

Since I own a compact SUV, following his example was not feasible due to the space available for storage. However, I did want a similar device, since I might be assigned a different area from my friend the next time we were deployed.

I began the design of my portable tower with a list of the criteria the design would have to meet.

1. It had to be at least 15 feet high to be of any real use. My thought was that if I could not increase the height of the antenna at least 3 times the height of the mobile antenna, then it wasn’t worth it.

2. The design had to break down into components no longer than 42 inches, since that was the space available in the back of my SUV.

3. It would have to be assembled and erected by one person, and be dis-assembled and stowed by one person.

4. It must be sufficiently rigid to support a reasonable antenna.

5. The antenna for the system must be multi-band capable, at least 2 meters, and 440. A co-located HF antenna would be a bonus.

6. It must be able to be deployed on or off pavement.

7. It must be reasonably light weight.

8. It must be made of easily obtainable and inexpensive materials.

The usual thought of an ex-military masting system was discarded on several grounds. It was too long, too expensive, and at least for me, not erectable by one person. I did not want to have to use guy ropes to hold the mast erect, and in any case that violated the one person criteria.

The material selected was Schedule 40 PVC conduit of 1 inch inside diameter. I chose conduit instead of PVC intended for plumbing because the conduit has the advantage of a built-in slip coupling on one end of each 10 foot section. This makes a more secure connection than the plumbing type

connector when cement is not used.

A total of seven 10 foot lengths of Schedule 40 PVC conduit were purchased, along with assorted hardware. Diagram 1.shows the finished components laid out in groups according to their function. Seven of the components were cut from the large end of each length of conduit. Another

four components were cut from the PVC stock, these did not require the expanded or swaged end. All eleven pieces so far were 42 inches in length.

The next group was the three components for the spreader bar assembly. These are 36 inches long. Another three components were cut 12 inches long. These were attached to one of the 42 inch components as shown in diagram 2. They are screwed to the longer PVC pipe, parallel and at 120 degree intervals around the pipe. This results in spacing which allows the three leg assemblies to lie parallel to, and in between the 12 inch spacers.

Two hose clamps of suitable diameter are then used to clamp the three leg assemblies to the center mast. This creates a rigid core with the leg assemblies coming off one end and the antenna mounted on the other end of the core.

The three pieces of conduit that were cut to 36 inches are attached to a central hub. I used an inexpensive closet flange available at the same store where I purchased the PVC conduit, but any material that allows you to bolt the three spreader bars to it spaced 120 degrees radially around the hub

would be acceptable. At the outer end of each spreader bar I secured a loop of galvanized strapping so as to allow the end of each leg assembly to be inserted and retained. If the tower is going to be erected on grass or dirt, the spreader assembly is not absolutely necessary, but it or some other method of securing the legs should be used if setting up on a paved area.

Each leg assembly consists of two components with the flared or swaged ends connected to one component without the flared ends, as shown in diagram 3. The central core unit has the antenna bearing component connected to it’s upper end. Because I used a discone type, VHF/UHF antenna, I had to add an 18 inch section of conduit to the top of the antenna

bearing component to allow the coax to exit the masting. The screws used to attach the spacers to the core prevent the coax from exiting through the bottom of the core.If you use a different antenna or mounting method, this may not be necessary. The radials for the discone antenna are stored in one

of the leg sections when disassembled. I leave the hub of the discone in place on the top section. The coax is left connected to the antenna hub and is stored with the other components.

In order to store the system properly, my wife made me a duffel bag-like sack with a drawstring closure that contains all of the components in an area about 10 inches in diameter and 42 inches long. The entire system stores behind the rear seat in my vehicle and still leaves room for all the other gear that I normally carry in what passes for a trunk area in my SUV.

The purpose of this project was to develop a transportable masting and antenna system for VHF and UHF that would satisfy the criteria set forth at the beginning of this article and provide a means to expand the coverage area of the radio beyond what was available through the normal mobile antenna. To that end it appears to succeed as well as I had hoped.

The total cost of the system was about 30 dollars. It requires no guy ropes or other securing in calm weather conditions, although it can easily be secured with any system of weights or tiedowns such as sand bags, tent stakes, concrete blocks or the like. Particularly when using HT’s in a simplex mode for a localized search or event, the use of a portable tower like this willfacilitate better and clearer communications with the radio connected to the antenna on the tower at the control point.

In addition, I discovered a very easy to build NVIS antenna for 2 through 10 MHz in a web search. (a) The tower will hold the far end of this wire antenna about 10 feet off the ground. The near end attaches directly to one of the antenna mounts on my vehicle. The only cost to add HF was the price of a 75 foot run of coax for VHF/UHF since the tower must be located at least 70 feet away from the vehicle to set up the NVIS antenna unless a convenient tree or other support is available.

This is, I am sure, only the first version of the tower. As the

system develops over time, modifications and changes will occur to tweak and improve the design. Already, HF capability has been added. This was considered worth the extra coax since I use an Icom IC-706MkIIG as the primary radio in the vehicle. If HF is unnecessary for a particular event or mission, then the tower can be erected right beside the car.

I am sure that there are easier ways to put up a temporary tower, particularly if one has storage space in their vehicle for regular masting, but if your space is as limited as mine, give the portable tower a try.

Member, Manatee County Amateur Radio Emergency Service.

(a) “Make a Quick, Easy, Cheap, NVIS Antenna for Roadside Operating”

By D.W. Thorne, K6SOJ, SEC, SV Section.