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Plessey Avionics antenna base 5820-99-647-9542 - WHIP needed!

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I hope someone can help. I have one of the antenna bases from Plessey. Reading around indicates it's for the VHF/UHF radios and the whip for it is a fibreglass centre fed one (coaxial bottom half and whip top section). I am deparately seeking one of these whips but cannot fund the NSN number for it. Actually, I want 2 of the whips as I intend to modify one of them (just the bottom section for the second whip would be fine).

 

I have been trying to get them from an ebay seller up north (Phased_001) in North Ridings or somewhere. But he won't post due to problems before, and also doesn't want me to arrange a courier to collect as he also had problems with that a while ago. I've also tried Drop Zone down here in Sussex but they don't have any at the moment. I see Woods Group Devon are selling the bases and whips together, but when I asked them if I could have just a whip, the chap said he's sure there are some there but wanted the whip NSN! I've also tried a company in Poland whip are selling the bases and they replied saying no whips....

 

So, please.... doesn't anyone have whips for sale?

 

I'm also coming up with a design for similar type of whip that I can use on HF, but I need to get hold of maybe just the screw ferule/connector from the bottom of the whip to screw into the mount. So even a trashed whip would be helpful.

 

Finally..... does anyone have any tech specs for the transformer/choke that's in this base? I know from putting it on my MiniVNA that there is a capacitor in the base, but don't know anything else about it!

 

All help will be gratefully received....

 

Cheers,

 

Pete

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Pete usually new items of equipment will be codified at the same time so I did a few searches going up & down the scale changing the last digit until the item went off topic. This is what I found:

 

5820-99-647-9541

Antenna

British Aerospace 6401-1-23578

Malvern Spec PT6036-0-600103

 

5820-99-647-9542

Tuner

 

5820-99-647-9543

Antenna

British Aerospace 6401-1-23583

Malvern Spec PT6036-0-611054

British Aerospace Spec PT6036-1-611001

 

So I don't know which "antenna" would be the one you are looking for. But there are some numbers to help widen your search.

 

The other likely source are the two people who traded as C&S Tat supplying radio stuff that people like to fit to their Rovers. They operate independently now, one is in Norfolk & the other in Somerset. I can give you contact details if you PM me.

 

73 de G4MBS

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Hi Clive. That's what I was hoping, that the NSN would be nearby. I'd be really grateful for the contact details of the fellas that were C&S Tat - please.

 

Thanks in advance,

Pete

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[ATTACH=CONFIG]88913[/ATTACH]

I have bases and antennas

 

Perfect - they're the puppies. I'll reply to your email at lunchtime and sort out PayPal for 2. Cheers!

Pete

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Colin (mattblack), I notice that the bottom tuner section isn't in the base in the picture. Mine still has it on. Have you ever disected one? I'd love to know exactly what's in it.

Pete

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hi Pete

I have never taken one apart, Its just one I grabbed from the shed from a box.

I see what you mean the others I have have the tunning rings ?

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I shouldn't think its going to be anything very exciting or complex.

 

Just a balun of some sort or a series tuned circuit (ie a capacitor in series with a modest inductance)

 

If you did dismantle one it might be difficult to glean much as it is probably embedded in epoxy or similar :(

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Clive

 

There is a very similar Racal tuner base which has a couple of relays and diodes to select matching networks based on a DC voltage fed up the centre of the coax - from memory the 30-80MHz range is split into 3 bands based on 0, 6 or 12v fed up the coax. Based on experience with the EVHF Clansman antenna which is also a sleeve dipole as this appears to be, but used with a a purely passive matching network (the "pineapple") a non adjustable matching network is (a) too lossy and (b) not very effective on all frequencies across such a wide range, so I'm guessing this tuner will be similar to the Racal one if they have any sense.

 

Iain

73 de G0OZS

 

PS Hopefully Colin (mattblack) can PM me with a price for a complete set so I can investigate further after pay-day ...

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Iain I didn't realise that. Yes of course a match across a wide frequency will be impossible to achieve without some internal complexities. My mindset is still stuck on my own operating habits over a narrow bandwidth.

 

Of course the item is called a 'tuner' not simply a matching unit as I had supposed.

 

The fun will dissecting it out from whatever it has been potted in. :D

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Thanks chaps. I'm wondering if this might not be anything complex. I know for sure there is a capacitor in series with the element. But I'm having a feeling that there might be something as simple as an unun. There's not much room and the concave base of the tuner ring on this doesn't leave much room for electronics. Also there's a breather/dessicant thing in the base marked with an M and made by Brownell ltd. This company make these things for keeping transformers dry.

 

Pete

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Pete, Clive

 

The Racal version MA972 which is also a flat disc is about the same size as the antenna base UnUn in Clansman VHF bases for use with the TUUAM - see ebay item 380863040910 for photos. The Plessey one isn't a badged MA972 as the Plessey antenna is a dipole rather than a rod as far as I can see. But the MA972 is an indication of the complexity possible in this form factor in the 1970s or early 1980s - it housed 6 or 7 coils plus capacitors and relays to provide separate tuning for three sub-bands.

 

The Clansman Pineapple did try to cover 30 to 76 MHz in one non-switched range and was only semi-successful - see http://www.g0ozs.org/clansman/EVHF/ - the match is only really good at about 33 and 60MHz, 50 and 70MHz being among the worst freqeuncies of all. To make it worse nearly 1/3 of the power vanishes in an attenuator designed to protect the radio from reflected power on the worst frequencies, so an EVHF driven at 50W is actually very little better than an elevated GSA operated at its limit of 25W.

 

Regards

 

Iain

73 de G0OZS

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BIG THANK YOU TO COLIN (MATTBLACK).Colin, the whips arrived safely the day before yesterday (Thursday). Very pleased - thank you very much.

 

Pete

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Mine arrived today (earlier than expected - thanks Colin) and I tried an MFJ antenna analyser directly connected to the input socket - strangely the only good match (1.5:1 and 50 0hms) was at 140-145MHz rather than the military VHF band 30-80MHz - I wonder if these were VHF air band or need power up the inside of the coax to work at all? the length of the whip is consistent with a 1/2 wave at VHF air band and the connector is an N-Type highly suited to higher VHF frequencies so I suspect the former. I'll perform a dismantling and photograph the insides of the tuner in due course.

 

I noticed the antenna element is labelled "MK87AA" in two places - I wonder if anyone knows what this means ?

 

Regards

 

Iain

73 de G0OZS

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Hi Iain.

That's also the frequency where I found a good match using the MiniVNA. I tried shoving 6V up the coax hoping I might hear something click, or maybe see some current draw, but there seems to be nothing. Putting ohmmeter across the terminal show completely open. I'm becoming more certain that there is just an RF choke or unun, perhaps on a core, and with a capacitor on the antenna feed. Will keep digging....

 

I did email Ian, M0YMK, who is chairman of the Vintage and Military ARS in the hope that he might know something, but he has never come across this antenna before.

 

73

Pete

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<snip>I'll perform a dismantling and photograph the insides of the tuner in due course.

 

I noticed the antenna element is labelled "MK87AA" in two places - I wonder if anyone knows what this means ?

 

Regards

 

Iain

73 de G0OZS

 

Hi Iain.

 

Have you carried out any dissection yet?

 

73

 

Pete M3KXZ

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I noticed the antenna element is labelled "MK87AA" in two places - I wonder if anyone knows what this means ?

I should think that is the ERM of the vehicle for which the item was issued.

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Pete

 

No, I have been busy preparing a new wiring harness for the SUMB I agreed to buy ! I will have a go this weekend sometime

 

Regards

 

Iain

73 de G0OZS

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Hi again Iain. I see from your web site that you've pulled apart a pineapple. Just wondering, is the thread atop the pineapple same as on this Plessey base?

 

For info, earlier on today I connected the Plessey base and fibreglass whip to my FT817. It's a pretty good match right across the 2m band. Crap on 70cm, reasonable match on 10m and lower but unsurprisingly deaf as a doorpost there!

 

73

Pete M3KXZ

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Pete

 

I think all my pineapples are in a container about 25 miles away so I can't measure to confirm, but I think the Plessey version is slightly larger and I think is triaxial (metal screw, middle and inner contacts) - certainly a coax rather than a single wire goes up inside the spring. The EVHF is definitely coaxial.

 

One of the screws holding the reddish-brown hard rubber cone onto the tuner unit of mine is very stubborn - I'm going to retreat for today and find a larger screw driver that fills the slot in the morning rather than wreck the slot in the screw head so further progress is halted for today.

 

Good that it behaves with actual RF power applied - hopefully all will be explained when I finally get inside !

 

Regards

 

Iain

73 de G0OZS

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I've now found a big enough screw driver to get the base open - internal photos at http://www.g0ozs.org/misc/Plessey_VHF_Base_Unit/ - it seems to be more of a fixed matching unit in the style of the EVHF or GSA bases than an ATU.

 

The coax input is connected to a coiled (1/4 wave?) coaxial stub around the rim of the base, and capacitively coupled to the inner of the coax going up to the antenna. This coax inside the insulated cone is coiled around a former presumably to suppress current on the outer. The aluminium disk seems to be a solid piece of Aluminium with nothing inside - all the components are mounted on tags and pillars under the cone.

 

I'll try to draw a diagram in due course.

 

Iain

73 de G0OZS

 

DSCN9331.jpg

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Iain, that's great. Did you find the base a pita to get off? I removed the screws but can't budge it, so need to use some force but don't want to damage it.

 

I'm thinking about building a different choke/transformer into the base and also modifying the whip so I can fit a section or two of my Clansman whips to the top. Bottom fibreglass whip section will remain coaxial so I'll end up with something like a vertical Zepp/"no counterpoise antenna". Intention is to have something to work reasonably from 20m-6m (possibly 30-6), i.e. I can get a good enough match to 50Ohm coax such that the 3 metre run to the TS480 won't be too lossy, and then use the tuner in the TS480. I'm using SG237 at the moment, positioned right at an antenna feedthrough for a whip, but would like to do without that so I can put it to use elsewhere. Also, it keeps draining my battery when I leave it connected. I figure that the losses incurred in a decent choke/transformer and short length of coax to the rig won't be significantly different from those incurred by the small inductors in the SG237 when matching weird loads!

 

73

Pete M3KXZ

 

I've now found a big enough screw driver to get the base open - internal photos at http://www.g0ozs.org/misc/Plessey_VHF_Base_Unit/ - it seems to be more of a fixed matching unit in the style of the EVHF or GSA bases than an ATU.

 

The coax input is connected to a coiled (1/4 wave?) coaxial stub around the rim of the base, and capacitively coupled to the inner of the coax going up to the antenna. This coax inside the insulated cone is coiled around a former presumably to suppress current on the outer. The aluminium disk seems to be a solid piece of Aluminium with nothing inside - all the components are mounted on tags and pillars under the cone.

 

I'll try to draw a diagram in due course.

 

Iain

73 de G0OZS

 

[ATTACH=CONFIG]90004[/ATTACH]

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Pete

 

>> Did you find the base a pita to get off?

 

After the one stubborn screw was out the base came off ok - a gentle sideways tap broke the seal. I think it is intended to be airtight and filled with nitrogen or dry air (note the filler plug underneath) so if pressure today is higher than the day it was filled it may be slightly vacuum sealed.

 

I am actually going to use mine for 2m VHF when I get the new SUMB operational later this year.

 

Regards

 

Iain

73 de G0OZS

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Thanks Iain. I'll whack it!

 

I'm not sure about the nitrogen or dry air - I was wondering if that plug might be a screw in desiccator to keep the innards dry. But I've just removed it and it's nothing but a plug so you're probably right. I expect the plug is inserted in dry air.

Edited by M3KXZ

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Pete

 

Certainly the Clansman and Larkspur radios were supposed to be filled with dry air (and I recall reading that base workshops had ovens for the purpose) before the dessicator plug was refitted. The aim is to minimise condensation from the air trapped within the set when the case is closed. As you have noted the plug on this antenna base also appears to be a dessicator.

 

Usually when I work on Clansman sets I try to close the case on a warm dry day in the sunshine rather than in the early hours of the morning in a cold damp shed for exactly this reason !

 

Regards

 

Iain

73 de G0OZS

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