Author Topic: DIY dipole CB base antenna  (Read 12054 times)

Offline Alan Georges

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DIY dipole CB base antenna
« on: October 22, 2011, 08:56:14 AM »
Since I started dabbling in CB again, I wanted a rugged, easy to store, easy to deploy base antenna that could reach out 10+ miles.  Cheap and DIY would be a pluses too.  A wire dipole could work, and there are several plans for these floating around on the net.

I built up this one: http://advancedsurvivalguide.com/2010/09/01/diy-build-a-portable-cb-base-antenna/   It worked, but I wasn’t entirely happy with it.  The connection between the braided shield wire and the ground leg was flimsy, adjustment at the connected ends made SWR tuning a pain, and when not strung up it’s a tangle-matic.  It does function though and the price is right, but there was a lot of room for improvement.

Improvements...  Being able to detach the coax feed line at a standard PL-259/SO-239 plug-and-socket would make setting up and stowing the thing easier, and make the junction between it and the dipole legs stronger.  Tuning the SWR by trimming the ends of the antenna would be easier than pulling it apart and re-crimping every step of the way.  And having some kind of storage for this rat’s nest of wires would help.

So here are the upgrades: mount an SO-239 on a plastic jar lid, and build in strain relief for the dipole legs so that they don’t yank loose from the connector.  Make the dipole legs a little longer than the standard quarter-wave 102” to give room to trim (105” is about right), and attach a paracord loop near each end to provide support independently of the ends that’ll need trimming.  Rig it so that the dipole legs can be stuffed into the matching jar for storage.  I used a 40oz plastic peanut butter jar, that’s what I had handy, but anything similar will work.  The lid is 3 1/2” wide, much smaller than that would be hard to work with, but the 40oz jar itself is a little large.

Parts list:
SO-239 chassis mount UHF connectors are about $1–$2 online.
20’ of 12 gauge hook-up wire was about $5.  Lighter 14 gauge will work as well.
50’ of coax with CB-standard PL-259 connectors on each end.  $20-$30.
Four 6-32 x 3/8” machine screws with nuts.
Matching #6 washers & lock washers are optional, but will make things stronger.
One eye lug that’ll crimp onto the lower dipole wire.
2 18” lengths of paracord.
Eight zip-ties.

For tools, all that’s needed are the basics.  Screwdriver, pliers, knife, the usual stuff.  Plus a soldering iron & solder (just one connection to make, to the center pin on the connector) and a  5/8” paddle bit ($4), to put a neat hole for the connector into the jar lid.  Will need a couple of smaller drill bits too, for the #6 screw holes in the lid, and another slightly larger than the antenna wires.

The only dimensions that are important are a 5/8” hole for the SO-239 connector and having the wire dipole legs originally cut to slightly longer than 102”.  The rest can be made to “about right, yeah that works” tolerances.

The first two pictures show how the connector is installed in the lid, and how the dipole legs are threaded through their holes to provide strain relief when this thing is hung up.  The only trick to drilling for the connector is to drill the main 5/8” hole first, put the connector in and drill one of the screw holes, put in that one screw to hold things steady, then drill the other 3 holes.  This keeps the remaining screws pretty well aligned.  A small drill press is best, but a hand drill will do almost as well.





The third picture shows how paracord is attached to the ends of the dipole legs with zip-ties.  Three hold it all together, while a fourth zip-tie marks where 102” is exactly.  Notice how the wire’s end can be trimmed to adjust the SWR without affecting how how the thing hangs up.



The last picture shows the completed antenna, tucked in for storage.  Don’t try to transmit with the wires all coiled up in the jar, it could fry your radio.



There are many ways to hang a dipole.  Sometimes they’re use horizontally at a half-wavelength above the ground for trying to shoot skip.  Here’s some info on doing it that way:  http://www.ventenna.com/files/Rad-Pattern.pdf  Then there are various “L” and “inverted V” configurations that give you some directionality,  http://www.tscm.com/radiapat.pdf gives an overview.  There’s a Marine field manual at http://www.everyspec.com/USMC/MCRP_6-22D_14371/ that has tons more on this, almost too much.

To get started I just wanted a omnidirectional pattern, so I hanged the driven leg (the wire connected to the center coax conductor) from a tree limb, and stretched it up vertically.  Some paracord and a tent peg on the ground end kept it straight and tight. Run the coax in as nearly perpendicular to the antenna as possible for at least 10’ or so to keep the signal from leaking back up the feed line.  Another length of paracord and another tree limb kept the coax properly placed.  I’d post pics, but the thing is almost invisible up in my oak tree.

It’s not really weathertight, but a plastic cone hat over the connectors could help a lot.  Will add one later.

About tuning, you’ll need an SWR meter.  Some radios have them built-in, which is what I used.  Hung up as described, I chopped the ends 1/2” at a time until the SWR reading got down to 1.5 and 1.7 on Ch. 1 and 40 respectively.  Good enough.  Cut the same amount off each leg at each step as you tune.  Mine came out to just over 102”.  If you’re operating in other frequency bands, you’re probably already a ham operator and know how to calculate the right length.

The antenna works pretty well for local comms.  Just after dark last Sunday evening, a friend and I with identical setups (this antenna with the bottom of the ground leg 4’ off the ground, box-stock Cobra 148 GTL’s, oak trees for support) were easily talking to each other at 10 miles using SSB mode.  Rolled up and stashed inside, it stays safe from high winds and is simple to put up after a hurricane passes.  Local communications problem solved, on to the next project.
Build it or buy it, start it up and try it, maybe even fry it.  Otherwise you'll never know if it works.

I swear, there are times it seems like "Baofeng" is Cantonese for "hot mess."

Offline Mr. Bill

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Re: DIY dipole CB base antenna
« Reply #1 on: October 22, 2011, 10:56:45 AM »
+1!  Excellent writeup -- thank you!

Offline Alan Georges

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Re: DIY dipole CB base antenna
« Reply #2 on: October 22, 2011, 12:48:43 PM »
You're welcome, and thanks for the kind words, Mr. Bill.  I'm having a lot of fun playing with this, seeing how far CB can be pushed while staying within legal power limits.  Will keep posting on how it works out, ranges and so forth, as things happen.
Build it or buy it, start it up and try it, maybe even fry it.  Otherwise you'll never know if it works.

I swear, there are times it seems like "Baofeng" is Cantonese for "hot mess."

Offline Alan Georges

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Re: DIY dipole CB base antenna
« Reply #3 on: October 25, 2011, 08:36:42 AM »
More radio testing over the weekend.  Could *barely* get through the same 10 miles as last weekend, using the same equipment and settings.  There was a slight difference in the weather, from formerly cool and bone-dry to cold with some light fog.  I swapped out my stock mic for a power mic (Cobra M75, $15) and friend on the other end reported a significant increase in clarity, as in “could hear you talking but couldn’t make it out” to “yeah, I can understand you now.”  So that was worthwhile.

Looking at line-of-sight calculations (http://en.wikipedia.org/wiki/Line-of-sight_propagation), D = sqrt(2*h), where h is the height of the antenna and D is 1/2 the distance between two similar antennas.  Using the center of the antenna as the height (4’ off the ground + 102”), D = sqrt(2*12.5’) = 5 miles.  Spot on for the 10 miles propagation we were getting.

It is gratifying to see this calculation confirm what we were actually getting in the real world, but it’s also a little depressing to see that reaching 20 miles LoS will take a 50’ mast -- more than I’m willing to put up.  But 11 meter AM isn’t LoS only.  The CB frequency band is a weird beast, and getting up out of most of the trees may make an even bigger difference.  Probably go to a base antenna on a mast pretty soon, will post any results from that as they come along.

Another factor is feed line loss.  Here’s an on-line calculator: http://www.arrg.us/pages/Loss-Calc.htm  What it tells me is that by switching out from crappy Radio Shack RG-58 to good LMR-400 on this 50’ feed line, it’s the same as upping my power by 20%!  Interesting too, because 12 watts / 1.2 = 10 watts, and we’re right back to the “a mile a watt” rule of thumb.

Summary:
  • The practical limit for over-the-tree-limb hanging a dipole is about 21’, which gives a LoS range of 10 miles, and this was confirmed in testing.  You can go higher with a slingshot and weight, but getting the feed line right becomes a hassle.
  • Getting out farther is going to take more altitude.  A better feed line will help too.
  • Feed line makes a big difference.  Cheap RG-58 gives a 20% hit on performance, but LMR-400 is stiff and will twist a simple wire antenna all over the sky.
  • Keep RG-58 for the the bug-out stowed antenna (it's more flexible), and save the good coax for a permanent base antenna.
  • A power mic can give you that last “oompfh” to get through when conditions are marginal, but don’t expect miracles.
  • Doing these tests is really easy when you have working cell phones to handle the "well that didn't work, I'm going to try this next" discussions.  It's good to get this all sorted out while other comms are working, I wouldn't want to sort this out after a hurricane when the cell towers are knocked out.
Mostly, I’m having fun and learning stuff.
Build it or buy it, start it up and try it, maybe even fry it.  Otherwise you'll never know if it works.

I swear, there are times it seems like "Baofeng" is Cantonese for "hot mess."

Offline backwoods_engineer

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Re: DIY dipole CB base antenna
« Reply #4 on: October 25, 2011, 09:54:10 AM »
Never mind my message.  I went back and read your original post.

Offline carpus

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Re: DIY dipole CB base antenna
« Reply #5 on: October 27, 2011, 12:41:38 PM »
Whenever I've built and used a di-pole antenna I use the 234/Freq=1/4 wavelength formula to figure out the length to cut to.

My best antenna was made from a stranded wire cut at 103 and 3/16"

my swr was never a problem and set up as a sloping di-pole I had a fun time talking to just north of niagra falls from here in the corn desert of central illinois.

I really like the peanut butter jar, that's outstanding.

I've been really kicking around the idea of getting back into CB and your post has made me want it that much more.. even for just the fun in building my own antennas again. Thanks
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Offline Smurf Hunter

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Re: DIY dipole CB base antenna
« Reply #6 on: October 27, 2011, 01:56:11 PM »
Cool. Time to sort through the old electronics junk pile in the garage and inventory some parts.

Aside from CB, I've got a 25-800mhz police scanner that I'm interesting in improving the range.  While it's portable with a 12v power supply it manly sits on my desk.  Is it possible to setup a single "one size fits most" antenna config for a variety of bands?

Even with the dinky OEM wand antenna that came with it I can receive HAM in the 400mhz range from 20 miles away.  Seems HAM and CB would be the freq. to monitor in a disaster situation.

Thanks for any advice.

Offline carpus

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Re: DIY dipole CB base antenna
« Reply #7 on: October 27, 2011, 02:52:03 PM »
with scanners being receive only there is a lot more you can do that will work.

generally the higher and/or longer the antenna the better.

with frequencies 30-300 mHz your getting into more LOS type stuff.
300mHz - 3gHz is mostly LOS

The more toward the strictly Line Of Sight signals you get the more your antenna height is the most important thing, the higher the antenna the farther the radio can "see".
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Offline idelphic

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Re: DIY dipole CB base antenna
« Reply #8 on: October 27, 2011, 08:48:26 PM »
Cool. Time to sort through the old electronics junk pile in the garage and inventory some parts.

Aside from CB, I've got a 25-800mhz police scanner that I'm interesting in improving the range.  While it's portable with a 12v power supply it manly sits on my desk.  Is it possible to setup a single "one size fits most" antenna config for a variety of bands?

Even with the dinky OEM wand antenna that came with it I can receive HAM in the 400mhz range from 20 miles away.  Seems HAM and CB would be the freq. to monitor in a disaster situation.

Thanks for any advice.
A fellow local amateur radio op uses his gutters.  They don't wrap completely around the house though, he also has used the chain link fence around his yard.. to receive you don't need to worry so much about the length of the antenna.  It's only when you go to put transmitted power to it that you need to worry.

If you use a tuner, then it will 'match' the antenna to the radio...  Everything affects how the antenna tunes.  Height, who wet or dry the ground is and how close objects are to it...

For CB radios - it might not matter... but for the higher dollar radios,.. do the math,.. I used zip cord to make a similar dipole, I've also used network cable..just pulled the pairs apart..
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Offline Smurf Hunter

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Re: DIY dipole CB base antenna
« Reply #9 on: October 28, 2011, 09:51:04 AM »
I'm working from my home office this morning and decided to tune into 441.6250 on the Uniden scanner.  Probably heard a dozen or so HAMs do their roll call from all over the country.  I'm in western WA, but heard calls from: CA, NV, AZ, TX, IN, LA.  My grandfather was a serious HAM, and hearing these old guys chat about nothing is comforting in an odd way.

While I hope we never find out, I wonder how many of these repeaters would remain functioning post SHTF.

Offline idelphic

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Re: DIY dipole CB base antenna
« Reply #10 on: October 28, 2011, 01:21:35 PM »
I'm working from my home office this morning and decided to tune into 441.6250 on the Uniden scanner.  Probably heard a dozen or so HAMs do their roll call from all over the country.  I'm in western WA, but heard calls from: CA, NV, AZ, TX, IN, LA.  My grandfather was a serious HAM, and hearing these old guys chat about nothing is comforting in an odd way.

While I hope we never find out, I wonder how many of these repeaters would remain functioning post SHTF.
Repeaters require the commercial mains,.. some may have battery backup, few have generator support.

Repeaters will be lost within a few hours of the grid going down.  Having a free standing comm system and the means to support it is critical.

You can build a small scale repeater with a few items,.. heck there is even a 'simplex repeater' module you can get and plug into some radios. You only need the module, one radio and the battery and antenna.  It'll be ok for 20 miles at best depending on radio, antenna and height.
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