HELP NEEDED: Design for a robust backpack-mounted 2m antenna

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HELP NEEDED: Design for a robust backpack-mounted 2m antenna

Post by crypto » Mon Jan 03, 2011 11:35 pm

What I'm looking to do is, in the next week, construct a robust (read: tough) 2m antenna that requires no ground plane, that can be mounted to my camping backpack or BOB.

What my initial thoughts are, are this:

6 inches of PVC pipe for the base, so that it can be threaded through some MOLLE loop and secured with velcro. If I use a whip antenna,

Weather-resistant construction, which probably (for simplicity's sake) just means a coax pigtail coming off the bottom that can be attached to the radio.

Metal retractable measuring tape construction wrapped in heat-shrink for the whip. I want to use the blade type steel tape because at 2m antenna lengths its reasonably rigid, and absolutely bomb-proof. I've discounted the idea of using fiberglass tent poles or fishing rods just because I know with my luck I'll snag it on a low-hanging tree branch. And, I can roll down the measuring tape into a much more compact shape than the poles will collapse down to anyway.

My game plan is to attach the metal tape to the PVC base with rivets to lock it in, then make a robust solder joint on the end to use as a feedpoint to the balun. Run the low-ohm side of the balun to the coax, and out the bottom of the PVC. Then I'd put endcaps on both ends of the PVC, fill the holes with silicone, and heatshrink the whole damn thing. probably put a SMA connector on the other end.



I absolutely don't want either a trailing counterpoise or radials on this thing, I dont want anything that can hang off it or snag. I will accept that there will be some performance penalty to be paid for that, but as long as I get significantly better gain than a rubber duck, with the above-mentioned bombproof construction, I will be happy.

So, hams and elmers: What do I need to be doing here? 1/4 wave? 1/2 wave? 5/8 wave?

Please don't get bogged down in the weeds here on whether I need a SMA or BNC or SO-239 on one end of the cable or another, or whether I should be using brass or aluminum rivets, right now I'm looking for big-picture guidance on how to make this project happen with a minimum amount of muss and fuss.

Thanks!

ETA: Lets figure on this antenna needing to accomodate a maximum TX power of 30 watts, to cover HT's and small battery-operated VHF amps. I don't need to be able to push 100w out of this thing :D
Last edited by crypto on Tue Jan 04, 2011 12:35 am, edited 1 time in total.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by bugoutvehicles.net » Mon Jan 03, 2011 11:50 pm

not a HAM guy or anything but i was a comm guy on my team in the Marines. Google a "jungle 292 antenna" and look for the field expedient type. if you give me a bit i can probably dig thhrough my MCI /Field Comm books and find something a bit more practicle if that doesnt work. We never traveled with anything more than the whip antenna or flexible tape while it was in the pack. The 10 ft whip was always broken down and secured. I dont see how you could possibly travel at any speed comfortably with a PVC antenna sticking out of your gear. When i was settled i could whip out a pretty good NVIS setup in just a few minutes.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Tue Jan 04, 2011 12:29 am

No, I think you misundersood what I was getting at. I'm talking about a PVC *base* about 6 inches long with the metal tape going out to whatever antenna wavelength I want.

Fuck a bunch of walking around with 3 feet of PVC pipe sticking out of my ruck

ETA: I'm not looking for a stationary antenna like a 292, I'm looking for something like the flexible blade antenna on a SINCGARS, only without the requirement that it work from 30MHz-3GHz. Even if I build a 5/8 wave antenna, for 2m it's only going to be about 4 feet long.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by Radio guy » Tue Jan 04, 2011 10:11 am

I have some experience working with pack mounted antennas for MBITRs, I made several prototype mounts to remote the stock antennas, field tested them and shipped to Iraq and Afghanistan before they became a commercial product.

Anyway, most antennas except for an end fed 1/2 wave will benefit from a ground plane or counterpoise and a half wave end fed is getting pretty long. I would recommend one of the full 19" length rubber covered 1/4 wave whips mounted high on the pack with an appropriate bracket and connector.

Since you have to use coax anyway you can parallel a 1/4 wave counterpoise wire with the coax feedline and heatshrink them together or use a 1/4 wavelength of braid over the coax grounded at the mount and heatshrink over that.

You might also benefit from a handful of #43mix ferrite beads over the coax right at the junction where the wire or braid counterpoise ends and heatshrink over that also.

If you must have a military looking tape whip there is a 19" long version made for the PRC-128 (P/N 37695-51 8026-1, have one in my hand) that terminates in a 5/16 thread. In this case you could use the small pyramid shaped antenna mount from a PRC-77 radio available on Ebay. You could probably cut down a longer PRC-77 tape whip to 19" and reinstall the top cap.
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crypto wrote:No, I think you misundersood what I was getting at. I'm talking about a PVC *base* about 6 inches long with the metal tape going out to whatever antenna wavelength I want.

Fuck a bunch of walking around with 3 feet of PVC pipe sticking out of my ruck

ETA: I'm not looking for a stationary antenna like a 292, I'm looking for something like the flexible blade antenna on a SINCGARS, only without the requirement that it work from 30MHz-3GHz. Even if I build a 5/8 wave antenna, for 2m it's only going to be about 4 feet long.

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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Tue Jan 04, 2011 10:34 am

Well, I'll be honest, I dont /have/ to have a military-looking tape whip. My logic was that the flexible tape whip was likely the most solid long-term solution to the problem, and one thats been in use for years. I was thinking that a 1/2 or 5/8 wave tape would be a higher-performing solution than a 1/4 wire whip.


Thats a brilliant idea about using a parallel counterpoise heatshrinked to the feed line. I will absolutely do that.

What would you recommend using for brackets and connectors? I was thinking of avoiding using connectors on the antenna end entirely, to aid in weatherproofing it.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by mattltm » Tue Jan 04, 2011 1:06 pm

How about this beauty...

http://www.armymars.net/ArmyMARS/Emerge ... NPACK.html

Maybe you could replace the fibreglass upper part with a SS whip or your tape idea?
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Tue Jan 04, 2011 1:26 pm

This is a cool idea, and in fact is the basis for my idea. I was wanting to simplify the construction, but it seems like an awesome idea. I'm betting I could in fact set up the metal blade antenna on the top section as he's done in lieu of a fiberglass whip.


Radio Guy: What do you think of this idea? Too complex? How do you think it would compare to a 1/4 wave vertical with a parallel counterpoise as you suggested?
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by TacAir » Tue Jan 04, 2011 5:58 pm

Use a AT-892 Tape whip with adapter from a PRC-77 , mount to ruck, cut to freq - you will need an antenna analyzer or spectrum analyzer to ensure a good match.

I use a TWA-2 with both an AT-271 10 ft whip (like is used with a PRC-77 or PRC-104) and an AS-1320/PRC47 15 foot with with the adapter - this lets me use a common CB mirror mount to allow the use of either of the antennas while the ruck sits on the gound.

Good luck.

(Edit) _ This assumes your ruck has a frame - AKA LC-2 or something metal.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Tue Jan 04, 2011 11:34 pm

Okay, well I've made a prototype with the following design changes:

it's now 1/4 wave with 2 counterpoises. :shock:

I used a small sealed project box, and put a BNC on top and bottom, and banana jacks on the sides.

The banana jacks are wired to the shield, and the center pins of the BNC's connect. Feed line goes in the bottom, counterpoises out to the sides, and the metal tape is on top.

I still havent trimmed it (I cant find my BNC-SMA adaptor) but it looks like it's going to be feasible.

If this works well, I'm going to rebuild it in a die-cast sealed box, use neoprene o-rings instead of rubber washers, apply, heatshrink on the blade, and use epoxy to connect the BNC male to the PVC whip base instead of a hot glue gun.

However, what I now have is a general purpose box with counterpoises on it, to which I can mount /any/ BNC antenna.

here's pics of what I have so far.
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The counterpoises do not even hang lower than the pack, so thats good news. I think they're about 4" too long as-is, so once I trim them to size, they're going to even less obtrusive. Since I've threaded them under the MOLLE, they simply arent the issue I thought they'd be.

The steel whip is very sturdy, it takes a bit of effort to make it fold over. using the clamps to wrap it to the PVC have given it a little extra stiffness, which I wasnt anticipating. It still folds and rolls smoothly though, so thats a win/win as far as I can see.

So, once I can hook this up to my SWR I'll tune it, and then see what I get for results compared to my big copper j-pole. I'm tentatively excited at the idea of carrying around the equivilent of a half-wave dipole on my back.

Total project cost so far is $30, including the tape measure and the 10 foot RG-58 patch I had to buy instead of the 5 foot I wanted.

Weight for the whole thing feel like its under half a pound, but I havent put it on a scale.


Did I mention I'm psyched that I can hook up any freaking antenna aerial I want? :D
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by Radio guy » Wed Jan 05, 2011 1:05 am

My MBITR mounts were just a double female TNC adapter and a Velcro Rip-Tie that was attached with a few small Ty-Raps to the adapter. The Rip-Tie would then wrap around a MOLLE loop or something high on a backpack so the 13" long VHF/UHF MBITR duckie antenna would sit mostly in the clear above the shoulder. The 2ft long coax jumper would wrap under the armpit and attach to the radio on the left front chest for right handed people, etc.

For the 3ft long VHF lo-band blade antenna they would sometimes attach the TNC adapter near belt level and snake the blade antenna up the back of a pack through some MOLLE loops until part of the antenna is exposed above the shoulders. This is lossy with much of the body in the way but the people using these needed to move in a hurry and are not like us hamsters who would rather have the best performance.

I tested the remote mounts in the 30-90MHz range with 3ft blade antenna and 3ft counterpoise wire mounted about mid way up a pack and the performance exceeded that of a radio hand held with antenna attached to the radio.

There are commercial versions of the remote mount and they don't use a counterpoise, the TCI MAST is one.
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crypto wrote:Well, I'll be honest, I dont /have/ to have a military-looking tape whip. My logic was that the flexible tape whip was likely the most solid long-term solution to the problem, and one thats been in use for years. I was thinking that a 1/2 or 5/8 wave tape would be a higher-performing solution than a 1/4 wire whip.


Thats a brilliant idea about using a parallel counterpoise heatshrinked to the feed line. I will absolutely do that.

What would you recommend using for brackets and connectors? I was thinking of avoiding using connectors on the antenna end entirely, to aid in weatherproofing it.

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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Wed Jan 05, 2011 1:17 am

When I make the next version of this, with the die-cast aluminum box, which is thinner but taller, I think I'm going to do the counterpoise braid running back down the feed line, as you suggested. It'd look cleaner.

What's the most rugged 50-ohm cable you know of? I know how to think in terms of attenuation, but not longevity. Since I'm guessing that you guys had cable survivability as a design requirement, can you offer any advice?
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by Radio guy » Wed Jan 05, 2011 9:43 am

I would recommend a good RG-58A/U or C/U with 19 strand center conductor and the older hard clear polyethylene dialectric, not the foam type. You would have to look at Belden, Alpha Wire or other mil suppliers instead of Radio Shack. The RG-223 double shielded version is also good and can sometimes use the same connectors as RG-58.

Adding strain reliefs to the connectors are probably more important than the cable, I use heavy wall adhesive lined heatshrink about 1 1/2" long over crimp type BNC or TNC connectors. If the radio is on your chest and the cable has to go up and make a turn down, a right angle connector will take some strain off the cable.
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crypto wrote:When I make the next version of this, with the die-cast aluminum box, which is thinner but taller, I think I'm going to do the counterpoise braid running back down the feed line, as you suggested. It'd look cleaner.

What's the most rugged 50-ohm cable you know of? I know how to think in terms of attenuation, but not longevity. Since I'm guessing that you guys had cable survivability as a design requirement, can you offer any advice?

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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Wed Jan 05, 2011 8:33 pm

I trimmed 2" off the steel tape, and the SWR meter now says 1.05. I'm gonna pronounce this a success.


Was able to hit a repeater on the other side of town, from inside the first floor of my brick house, using 5W.

I am happy.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by Radio guy » Thu Jan 06, 2011 9:04 am

Sounds like a winner. How bout covering the tape with some thin heatshrink like the MBITR 1M blade antenna? If you check the match in the 440MHZ band it should be usable there also and you could add another tape element around 12" long for 220MHz if you isolate it with heatshrink then heatshrink the whole thing.
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crypto wrote:I trimmed 2" off the steel tape, and the SWR meter now says 1.05. I'm gonna pronounce this a success.


Was able to hit a repeater on the other side of town, from inside the first floor of my brick house, using 5W.

I am happy.

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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by aa1pr » Thu Jan 06, 2011 9:33 am

what am I missing a 1/4 wave is only approx 19 inches?

Build a coaxial sleeve dipole & insert that into the pvc, no need for ground or tuning

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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Thu Jan 06, 2011 10:01 am

Radio guy wrote:Sounds like a winner. How bout covering the tape with some thin heatshrink like the MBITR 1M blade antenna? If you check the match in the 440MHZ band it should be usable there also and you could add another tape element around 12" long for 220MHz if you isolate it with heatshrink then heatshrink the whole thing.
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Yea, I bought 5 feet of 1" heatshrink for this express purpose. I need to re-work the antenna mount before I use it though, that shit is expensive. :D

Unfortunately neither any of my friends nor I have a 220-capable radio, but I may do that anyway. Thats a good idea, thanks!
Last edited by crypto on Thu Jan 06, 2011 10:10 am, edited 1 time in total.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Thu Jan 06, 2011 10:02 am

aa1pr wrote:what am I missing a 1/4 wave is only approx 19 inches?

Build a coaxial sleeve dipole & insert that into the pvc, no need for ground or tuning

It's now a half-wave. It worked so well at half-wave I didnt want to trim any further. :D
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by Radio guy » Thu Jan 06, 2011 1:30 pm

Did you make a J-pole style matching section at the base or something similar? A half wave end fed antenna is very high impedance and will not directly match to 50ohm coax.
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crypto wrote:
aa1pr wrote:what am I missing a 1/4 wave is only approx 19 inches?

Build a coaxial sleeve dipole & insert that into the pvc, no need for ground or tuning

It's now a half-wave. It worked so well at half-wave I didnt want to trim any further. :D

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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Thu Jan 06, 2011 1:55 pm

No, I threw the 2 counterpoises on it.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by Radio guy » Fri Jan 07, 2011 10:00 am

Interesting... Some things to consider when making antennas is a resonant center fed half wave dipole or quarter wave whip (and 3/4 wave whip) against a groundplane will match directly to 50 ohm coax but most anything else like a half wave or 5/8 wave end fed will require some matching considerations.

If you have a half wave end fed whip thingee that shows a good match then you lucked out with some odd combination of coax length and other factors that is fooling the SWR meter. Or the meter is a CB type and not giving accurate readings at VHF, etc.

If it works and your happy I guess that's the bottom line but I would want to know why it shows a good match and/or redesign it to something that is technically correct, cuz that's how I roll.
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crypto wrote:No, I threw the 2 counterpoises on it.

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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by crypto » Fri Jan 07, 2011 10:35 am

Well, I'll be honest here, I was expecting to have to trim another foot off it. Before I started trimming, I put it on the meter (which is an old realistic SWR-meter I've had for a long time, and used to build all the 2m antennas I've made) and after calibrating it, SWR was extremely high, approaching 3.5:1 at 146MHz.

I took off half-inch increments and saw a large drop in reflected power each time, after 4 cuts the needle barely moved when I keyed up on 146.

So, my tape blade is currently 33 inches, with another few inches of connecting wire and connectors going back to where the feed line splits off.

The counterpoises are also longer than 1/4 wave. They're about 24 inches long each. they were going to be the last thing I trimmed, but when I got the results I did, I decided to leave well enough alone.

So, here's my question. If it makes the SWR happy, and performs well (I was able to work a cross-town repeater with it from inside my house, which is something I normally have to use my large copper pipe J-pole to do) , I'd rather understand why this one works well, when the numbers don't add up, so that when I make the next one (this was a prototype after all) I can improve it.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m ant

Post by nacho » Tue Jan 11, 2011 3:21 am

This is a really cool idea, it got me thinking.
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Re: HELP NEEDED: Design for a robust backpack-mounted 2m antenna

Post by mjolniractual » Tue Oct 02, 2018 11:19 am

Did you ever get this running? If so, could I trouble you for a build sheet of some sort for it?

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