Saved a couple of Larsen PO150 VHF antennas with ground planes from the dumpster a few days ago.

We were wondering if we could repurpose them to 1.25 or 70cm by changing the length of the whip.

Are you sure they are UHF?

And are they true quarter-waves with no matching arrangement in the bases?

The PO150 is a 5/8-wavelength for the VHF band from 144-174Mhz.

Being a 5/8-wave, there's a loading coil in the base and you can't get by just trimming the whip.  The coil would have to be modified to retune it to another band, which probably isn't very feasible if it's sealed into that base part.

I believe the uhf version is the PO450.
Edit: @Gamma762  

It will work well for six meters as is.

Definitely VHF, was thinking UHF when I typed it.

@lorazepam

@D_Man

Good info there. Thanks.

We'll just keep them VHF then.

Check it with an analyzer, much depends upon whether the element was ever tuned above the 2M ham band.
73,
Rob

ETA: I had an old Larson that presented a fair load on 70cm. (Sold as a 2M single band antenna.) Being relatively close to the repeater it worked fine, even from below a river bluff, with an HT in the car.

Could use it for MURS, or VHF Marine/Railroad monitoring with a little tuning.

I've got my NanoV4 with me so we'll sit down and see what we can get with it.

Good save.

It's an LMR type VHF antenna, usable for 2m through 174mhz depending on whip length.

For 2m you'll probably need a 48" whip or thereabouts. Maybe 48 1/2. I usually end up taking off about 1/2 or 3/4 inch from an NMO150.

Antenna analyzer is of course very helpful but here's the Larsen cutting chart:
https://www.repeater-builder.com/antenna/pdf/larsencuttingcharts.pdf

PO150 is on page 13. If you measure the whip the chart will get you relatively close to what it was cut for.

The color & details can give you an idea of how old the antenna is. If you post a pic of it we could estimate the vintage

The nut on top of the plastic base is threaded, normal right hand thread if you want to remove it, can also loosen the little set screw and remove the whip from the nut. Some of them are hollow so that the whip can extend down into the base slightly which makes tuning easier.

As far as another band, as previously mentioned that base has a coil inside which would probably make it unsuitable for higher frequency operation, but it would work on 6 meters if you're willing to experiment with whip length. Different brands 5/8 wave mobile antennas generally resonate somewhere in the 6m band as a slightly base loaded 1/4 wave, but getting them to work as a dual-band antenna can be a challenge. Magnet mount vs permanent mount alters the 6m tuning noticeably (as with any 6m mobile antenna).

@Gamma762

This is from a system that was installed when there was another organization on ground and installed on one of their modular buildings (trailers). So at least 10+ years old and probably closer to 15.

Attached File


The whips are 41.25 and the other is 42.25. Ground plane whips are around 12 inches long each. The Larsen cutting chart you linked says 166mHz for the whip length. Scratching my brain to figure out who might have operated on that freq.

The nut holding the whip is through on one of them and not the other. One is a stainless thread on the coil and the other is brass. Galled the brass thread but easy clean up with a die. The coil is about 1.6-1.7 inches long and a SO239.

I don't know the use for these when originally installed but generally you see them in a set of 4 with the same antenna and ground plane so I'm thinking a base/repeater setup. I recall seeing 4 on the demolished CHU but haven't seen the other 2 hit the dumpster yet.

I've got a nanoVNA here to play around with, so I'll check it out and see where they're at when I get some free time.

Feds... maybe Railroad. Or ~15 years ago was the era of LoJack, which nominally operated on a 170-something frequency. Direction finding receivers would typically be a set of four antennas in a square and this was the common setup for LoJack. The new "Positive Train Control" railroad stuff on 216MHz has replaced some older stuff that worked on the more common 160ish railroad frequencies. If it was four antennas in a square it was someone's direction finding receive station. If you're in or overlooking a big city, probably lojack. If you're overlooking a large body of water, maybe CG.

Pretty nice adapter for the ground radials, I haven't seen that one before. It's not the one that Larsen sells.

I'm in Iraq so there is no telling who was using it for comms. And IIRC, these were mounted in a line along the building edge and not in a square.

Larsen apparently still makes the kit for the ground radials: Larsen BSAKit . Just having it in my hands and looking at it doesn't seem like it's worth $97. Very simple but neat add-on though that you could probably make in a machine shop for less than a quarter of the price. Cheaper if you're running your own equipment.

Lots of older but still neat stuff like this just left rotting these days due to the upgraded stuff they use now. Just the miles of coax installed and then abandoned over the years in this place alone is staggering.

EDIT - And in case anyone is wondering, there is a somewhat active amateur radio presence in the region for the locals, and you can occasionally get a QSO from within the region. But the government here really frowns on foreign amateurs operating in their country so most of us stick to SDRs for entertainment.

Wow, larsen is sure proud of their GPK.

You can find something really similar made by Jetstream or Nagoya for 1/2 to 1/3 the price.

If you want 2M, I'm sure replacement whips are available. Just be sure to get the right length.
73,
Rob

Oh. Well then. Yeah no telling, probably mil or contractors that were using COTS two-way equipment. 160ish range would be a good place to do that in Iraq.

So probably just multiple radios then. I don't think there is any MiMo equipment made for 160MHz.

I see that's a standard Larsen base adapter kit but I've literally not seen that exact one before. The kits I have seen have a pretty flimsy round piece that holds the ground radials. Maybe I'm thinking of another brand or something I dunno.

If I was in Iraq I would figure out some way to get on to the Qo-100 satellite. 2.4GHz uplink, 10GHz downlink geosynchronous satellite that covers europe, africa and the mideast.  Adalm-Pluto SDR is apparently the hot ticket for DIYers... downconverters are readily available or can be repurposed from TV reception dishes, a 1 meter size dish is good apparently. Pluto can TX on 2.4GHz so just need a filter and amplifier if you had the ok for transmit.

It's entirely possible that in the piles of abandoned stuff lying around that one could come up with a dish, a 2.4GHz dish feed, and an X band LNA/downconverter. If you're content to listen just the reception part would be a neat accomplishment.

The Larsen GPK is pretty bomb proof (no pun intended) from its construction. Steel base with the same material for the radials as the whip is, and all held in by 2 set screws. Standard SO239 running into it.

I've seen the Pluto on RTL-SDR but haven't researched it much as I'm still playing with a basic v4 and nanoVNA. And didn't know the Pluto could Tx so looks like I have another rabbit hole to keep me busy.

I could probably Macguyver something together to hit that sat if I had enough time to collect the right parts, but we're limited to the basics. Anything more than that and the sigint people get a bit weird. I watched them almost have an aneurysm when they found out a friend of mine put up a wire and was doing CW with his MCHF, so we just keep it simple now.

I'm amazed at what my V4 can do. For $30 it's outrageous.

If you're ok for local FM QSOs, maybe you could work the ISS repeater or other FM satellite.

If the Pluto covered down to 500khz they'd sell a billion of them.