Feature Article
by... Wayne Sarosi - KB4YLY |
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My first low budget antenna is a simple ground plane antenna and since 2m and 70cm FM are the most popular we'll start there.
I have three designs which comprise area and they vary in pricing that won't bust your wallet.
The common component is a Radio Shack SO-239 chassis mount female connector. This fits a PL-259 UHF male cable connector. You can use N type chassis and cable connectors for UHF if you please.
Antenna #1: You'll need some brass brazing rod. Nothing really thick, just big enough to fit into the chassis mount center pin for soldering. Why brass? It sol ders easy. You can use copper solid wire also. However, the brass is a bit more stiff.
File down the mounting holes so that the brass undercoating is exposed. Cut the brass or copper to 5% over a quarter wave:
[(246/fMHz) + ((246/fMHz)*.05)] = ground plane elements
Cut four pieces and solder them to the chassis mount mounting holes so that they are perpendicular to the frame of the connector:
Using a pair of needle nose pliers, at the solder joint, just below the chassis connector, bend the ground plane elements out to between 45 and 50 degrees.
Now cut the driven element to the same length and solder into the center pin. Attach your coax. Set up this tuning example:
Antenna --- coax --- SWR meter --- rig
Use low power. You can attach the antenna with a hose clamp to a broom handle for tuning via the coax connection. Go to the low end of your rig and transmit. Note the SWR reading. Do the same at the mid point of the band and the high end.
If the reading is better low than high it's too long and needs to be trimmed. That should be the case. Cut the top tip down a tad bit at a time and re-measure the SWR until the center of the band or your desired portion is as low as it can be. If it starts to go back up in SWR stop. Remember that in 70cm FM the input is mostly at the upper end of the band for repeaters. Thus the SWR should be best on the transmit, not the receive.
Repeater is 443.1 MHz. Your transmit frequency is 448.1 MHz so why tune to the repeater frequency? If you are going to operate at 446 MHz, then tune the antenna there.
If the SWR is better High than Low, it's too short; thus re-cut a new longer element, remove the first one and solder the new one in place. Repeat procedure. If it's a 'right on' SWR, leave it.
Mounting is a breeze with the hose clamp and vertical whatever like a rod, broom handle, angle iron, your choice ... Clamp between the coax connector and the 'whatever' you choose and then clamp the whatever to the tower or tower arm:
Cost: About $3 for the Chassis mount connector, Brazing rod comes in 3' lengths and your need three for 2m and two for 70cm. About $.50 each. And a hose clamp ab out $.75. OW! that hurt my wallet! ;-) How long do they last? I have two up n ow for nine years!
Antenna #2: This one is made from a tin funnel, a small piece of copper wire or brass brazing rod and an SO-239 chassis mount connector.
Now, one must realize that tin funnels do not come in 2m size so this design is for 70cm FM.
Cut the narrow end off the tin funnel so that the chassis connector's threads will just fit into the hole of the funnel. Once this is completed make four short slices into the newly made hole area so that the corners of the chassis mount connector can wedge into place. Prepare the surface of the SO-239 by removing the outer coating so that the brass is exposed and solder tin these areas. Wedge the chassis connector in place. Bend the extra tin over the chassis connector's mounting edge and solder. Insert the quarter wave driven element into the center pin, cut and tune as explained in part one.
Mounting is right off the tin funnel with a small piece of [your choice]:
Coax goes up the middle like a discone.
NOTE: Paint the unit since tin will rust. Use a non-conductive paint.
Cost: Tin funnel $1-$2, piece of brass brazing rod $Scrounge, SO-239 around $3.
Antenna #3: Similar to antenna #2, brass brazing rods or solid copper wire can be soldered to the funnel and a longer driven element can be added for 1.25 and 2m.