150-160mhz. diy. ground plane radials length?

crippledchicken · #1 (**permalink**) 01-02-2009, 03:10 PM

I plan to make one of the ground plane antennas using the so239 connector and was wondering if someone could give me an idea of what would be the best length for the radials? I will probably be using #12 gauge wire. any other suggestions are welcomed. I'm just looking for something cheap, and simple to build. Thanks!

N_Jay · #2 (**permalink**) 01-02-2009, 03:34 PM

1/4 wave at the lowest frequency you want plus about 5%.

kb2vxa · #3 (**permalink**) 01-02-2009, 04:06 PM

Funny you should ask, the SO-239 ground plane is one of my specialties so I guess rather than refer you to a hundred previous threads I'll go through it again, briefly. (?)

OK, the radials are a small percentage longer than the radiator (not critical) so about 1/4" should do for putting a small hook in one end where it passes through the hole in the flange. The actual increased length is the distance from that hole to the center so you're OK there. Yeah, soldering is better than nuts and bolts.

For indoor use it's 14ga solid copper, 12ga is too large a diameter to fit into the center pin and I guess you figured out a good source is an old piece of Romex attacked with a sharp knife. For outdoor use it's 1/6" diameter brazing rod, soft copper wire bends easily in a strong wind. File the ends of the wire or rod and the solder blobs nearly flush with the bottom of the flange, more on that later. Then a gob of silicone bath tub caulk seals the gaps between the center pin and the insulating bushing and it and the main body of the connector.

Now you're ready to mount the wee beastie. Indoors a short length of 1 1/4" diameter pipe will hold it nicely, what to do with it next is for you to figure out. Outdoors it's ordinary TV antenna mast and whatever sort of brackets suit how you want to install it. Here's where it gets interesting, shove the coax through the mast before you put it up and screw it to the antenna just letting in hang on a couple feet of slack for now.

Next you waterproof it the good old fashioned Army way using a roll of cheap, like Wally Mort black vinyl electrical tape because it stretches a whole lot better than Scotch or the other expensive stuff. (I wonder if the Army figured that out.) Starting a couple of inches down the coax below the connector spiral wrap the tape stretching and holding tension as you go overlapping each turn at it's center (half the width of the tape) making sure it's all nice and tight, flat and even. When you get to the flange wrap a couple of turns right up against it (not over it dum dum) and start your spiral back down to just below where you started. Then wrap a couple of slack turns, that is neither stretched nor tensioned straight around the coax, snip and you're done. The idea behind those last turns is the tape tends to shrink back and separate so if you keep tension to the end you'll have a few inches of tape flapping in the breeze eventually.

I guess by now you figured out that by pulling the coax back down the mast the antenna flips up top and sits there looking all kind of cute without any nuts, bolts, baling wire, duct tape or other redneck means of holding it there. Remember filing off the excess radial stubs and solder blobs? Bubba forgot to do it and now his antenna sits cockeyed, he'll still be trying to figure it out a month from next Sunday so will one of you please give him a link to this thread?

One last thought, the radials should be at 90 degrees to the radiator or bent >slightly< downward, any more and it ceases to be a ground plane and becomes a coaxial dipole.

smason · #4 (**permalink**) 01-02-2009, 04:49 PM

Quote:

Originally Posted by kb2vxa

One last thought, the radials should be at 90 degrees to the radiator or bent >slightly< downward, any more and it ceases to be a ground plane and becomes a coaxial dipole.

Don't "they" say that bending the radials to 45 degrees make the impedance closer to 50 ohms?

crippledchicken · #5 (**permalink**) 01-02-2009, 08:50 PM

Thanks for the detailed info I'll see what I can come up with!

elocutionist · #6 (**permalink**) 01-02-2009, 09:54 PM

I've built a few of these over the years, but the guide I was following from the ARRL Handbook has the radials cut slightly shorter than the radiating element. I bend my radials down to 45° to start with and then tweak them up or down to get a 50 ohm match. An ARRL article showing the dimensions for some ham bands is at http://www.arrl.org/tis/info/pdf/ab18-16.pdf.

For 155 MHz I calculate a quarter wavelength at 19", using the simplified formula 2952/frequency for inches.

Christopher

kb2vxa · #7 (**permalink**) 01-03-2009, 04:52 PM

"Don't "they" say that bending the radials to 45 degrees make the impedance closer to 50 ohms?"

"I bend my radials down to 45° to start with and then tweak them up or down to get a 50 ohm match."

There's your answer, sort of. What the Electrocutionist doesn't say is where in the line he put the SWR bridge or antenna analyzer. I bent mine down slightly but not to 45 degrees and got a perfect match every time. That's the nature of a ground plane, it's not too fussy unless you bend them down so far it turns into a dipole and exhibits the characteristic 72 ohms impedance. Maybe he was just being picky or didn't put the meter at the antenna where it's supposed to be when tuning and tweaking. Putting it at the transmitter can give false readings that sometimes can be made up for by fiddling with the antenna but it's still a poor match, you're only fooling the meter.

Hey Chris, I WROTE the darn book and the title is The Radio Amateur's Handbook published by the ARRL. (;->)

prcguy · #8 (**permalink**) 01-03-2009, 08:46 PM

I have never had to bend radials down to achieve a perfect match on a 1/4 wave ground plane. When you put a 1/4 wave whip on your car do you need to hammer the trunk lid into a cone shape at the antenna mount to get the thing to match? I don't think so. The +5% rule on ground radials has been proven to be of no value, make them a calculated 1/4 wavelength and trim the vertical element for best match. The more ground radials the better, 4 is better than 3, 8 is better than 4 and a solid sheet is best even though the improvements are very small after about 4.
prcguy

majoco · #9 (**permalink**) 01-04-2009, 04:20 AM

Trimming the vertical element of a quarter wave antenna to match 50ohms using a VSWR meter as an indicator is a fallacy. All you have done is make the antenna a complex impedance (R + Xl or Xc) which to the "resistive only" VSWR indicator "looks" like 50ohms. In fact it is the resultant of the real and imaginary impedances. A true quarter wave antenna over a perfectly conducting infinite sheet is going to show 72ohms.

prcguy · #10 (**permalink**) 01-04-2009, 12:29 PM

Wow, this is new information that a quarter wave whip over an infinate ground plane is 72ohms, where did you get it? We were all taught the theoretical impedance of a quarter wave whip over infinate ground plane was around 35ohms. I have measured this with an HP 8510 vector network analyzer and it was under 50ohms but close enough. I guess my mistake was the ground plane was not infinite, just much larger than a quarter wavelength on all sides, sorry. All those times I dragged a vector analyzer to the car and played with antennas must have been a waist of time because I did not measure anything like you are claiming.
If a half wave dipole in free space is around 72ohms then please tell us how the impedance of a quarter wave vertical over an infinite ground plane can be the same with all the extra capacitance between the whip and ground? If the quarter wave vertical is 72ohms then a dipole in free space would have to be around 120 to 150ohms. Boy, I guess we have been wrong all these years and our test equipment must be way out of cal. And please tell us what you use to tune your antennas if a VSWR meter is not good enough. If Xc, Xl and R are such that there is no reflected power in a 50ohm system, then what else do we do? Please help us with limited antenna smarts.
prcguy

Quote:

Originally Posted by majoco

Trimming the vertical element of a quarter wave antenna to match 50ohms using a VSWR meter as an indicator is a fallacy. All you have done is make the antenna a complex impedance (R + Xl or Xc) which to the "resistive only" VSWR indicator "looks" like 50ohms. In fact it is the resultant of the real and imaginary impedances. A true quarter wave antenna over a perfectly conducting infinite sheet is going to show 72ohms.

majoco · #11 (**permalink**) 01-04-2009, 12:41 PM

Sorry, Guys, I'll grovel for forgiveness! It was late at night, I'd been out to a BBQ with the required amount of brown sparkling health beverage! Fingers going faster than brain.

I meant to say....72/2 = 36 ohms.....

There's nothing wrong with your test equipment!

prcguy · #12 (**permalink**) 01-04-2009, 12:58 PM

Oh thank you . I was trying to stuff my test equipment in a trash can but it was not going to fit.
prcguy

Quote:

Originally Posted by majoco

Sorry, Guys, I'll grovel for forgiveness! It was late at night, I'd been out to a BBQ with the required amount of brown sparkling health beverage! Fingers going faster than brain.

I meant to say....72/2 = 36 ohms.....

There's nothing wrong with your test equipment!

kb2vxa · #13 (**permalink**) 01-04-2009, 01:29 PM

Hey guys why all the brouhaha over a silly ground plane? Just build it, it works and leave it at that. Hey, I once knew a guy who banged his car into the shape of a cone but he was from the planet Remulac. MEBS! MEBS!

crippledchicken · #14 (**permalink**) 01-04-2009, 05:55 PM

The one I built a couple years ago I think I cut the radiator to 19" and 18" I believe, is what I cut the radials to It seemed to work much better, than the simple coax with 18" of jacket removed and trying to separate all the braid so it could just hang down as someone recommended to me a few years ago lol! but, I live in a mobile home and they just won't allow anything hardly, to be received through all the aluminum a real pain in the butt. I wanted to get an so239 connector with the Teflon insulator so I wouldn't have to worry as much about soldering heat but, that's all the local RS carries anymore so, I may have to use screws. any more they seem to be more of a Cell Phone place than electronics. any way, I'm going to try to make my ground plane tomorrow and try to hang it outside my bedroom window and hook up my pro-163 in there. Thanks!

jonny290 · #15 (**permalink**) 01-04-2009, 09:05 PM

I've had vertical dipoles for 2m that are dead on 1:1, and I've had flat-ground-plane verticals that did the same thing. I just shrug and plod on.

According to my models (I have been working on a tweaked dual-band SO239-based ground plane), 120-135 degrees seems to be the sweet spot for pure 50 ohms resistive. For my dual-band, I have three 2m radials at 135 degrees and three 70cm radials at 120 degrees from vertical (the 70cm elements are rotated by 60 degrees). It shows 48-52 ohms for both bands at the resonant frequencies and less than 2dB pattern irregularity from true omni on both bands.

kb2vxa · #16 (**permalink**) 01-05-2009, 06:25 AM

Oh boy you sure are a cripple chicken if you buy from Rat Shop! (;->) If you're melting the connectors you're obviously WAY overheating them, I suspect a crappy Weller 8100 or 8200 gun. Get a 100W dog leg industrial soldering iron (Hexicon is excellent) and learn how to solder. Meanwhile, Amphenol still makes PL-259s and SO-239s with good old fashioned low loss Steatite insulators, melamine termoplastic won't melt or burn. (They make ash trays out of it.)

crippledchicken · #17 (**permalink**) 01-05-2009, 11:03 AM

Only place (local) to buy from not worth trouble to order and,I know how to solder been doing it for years even SMDs and yes, I really am crippled I have one leg missing at the hip! does that make your day now?

N_Jay · #18 (**permalink**) 01-05-2009, 11:17 AM

Quote:

Originally Posted by crippledchicken

Only place (local) to buy from not worth trouble to order and,I know how to solder been doing it for years even SMDs and yes, I really am crippled I have one leg missing at the hip! does that make your day now?

Warren missed his sensitivity training class. Seems it was on the same day as "how to use the Quote feature" was taught.

smason · #19 (**permalink**) 01-05-2009, 01:15 PM

Ok, what is this 50 ohms "resistive" that everyonme is talking about??

I thought that for an antenna to resonate at a given frequency, involves inductance and capacitance.
The only resistive load I've ever knowingly uses was a dummy load.
Maybe I need to go back to school

DDan · #20 (**permalink**) 01-05-2009, 02:04 PM

Quote:

Originally Posted by smason

Ok, what is this 50 ohms "resistive" that everyonme is talking about??

I thought that for an antenna to resonate at a given frequency, involves inductance and capacitance.
The only resistive load I've ever knowingly uses was a dummy load.
Maybe I need to go back to school

A proper antenna will be neither capacitive nor inductive at the desired frequency. The antenna will be resonant so it will be a resistive load only.