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Antenna base query


Stone

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Hi all,

 

Our new RB44 has what looks like a Clansman antenna base on it (like on the front wings of Landies) but it's been mutilated by the previous owners and had top and bottom snipped off :(

 

The designation marked on it is 'Base Antenna Support Number 31, mk7' with a NSN of 5985-99-744-3410.

 

Four questions:

1) Is it actually from Clansman or just a lookalike?

2) Is it supposed to have a fixed antenna element or does the operator slot the right length into the top?

3) Can I get an antenna with this base and a sensibly-common RF connector on it? (N-type would be great but I suppose I'll get what I'm given...:D)

4) If not has anyone got a spare I could buy off them?

 

Thanks in advance!

 

Stone

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Perfick, thank you! :D

 

Is the lead that attaches to the radio part of this mount or attached separately? I can't tell on ours if it's bolted onto the inside of the mount or something...I assume there's an electrical connection between the bottom end of the inserted element and the mount when it's screwed in but I don't know how that gets into the radio.

 

Also would this fit it? It would be easiest as a springy single-element whip but I guess they're designed to be taken off as necessary.

 

At least if I have a spare I can bodge our own antennas onto the hole pattern as necessary :-)

 

Stone

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Hi Stone. To try and answer one or two of your questions.

Firstly what are you wanting to do? One , plenty of these bases about, yes ebay ok but be careful. Good dealers about, just had a look and found some at very good price plus postage.

Dont want to get too involved with the differences between HF and VHF radio's (Dont even know what a RB44 is, sorry) But it does make a vast difference how a antenna is connected to a radio.

This base simply has antenna elements dropped in it and clamped, for VHF it is 2x1mt (as on Landies) but if for HF can be up to 4x1mt.

I would suspect that if you have only this base it has been used for HF and the antenna lead is connected via. gland/screw with a BNC on the other end connected to a TURF/SURF (depends how many HF sets used!

If for VHF a, to put it simpley, a further app. 2" ring is fitted between the base and vehicle and from this ring a cable is connected to a TUAMM with a BNC at each end.

Hope of some help and I do stand corrected.

Andy.

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Thanks for that, I've now found this pic (described as Base Antenna Support No31 Mk3 with Base, Antenna Element):

T.jpg

 

The connector has been cut off ours so I couldn't see what it used to have. It doesn't look like normal coax cable though, it appears to be just plain stranded copper wire...I don't really understand how you could sensibly terminate it in a BNC as they only make sense with a central conductor and outer screen.

 

Is the spacer there to attach the cable screen to the top of the hull while feeding the central conductor into the antenna element? Can't see how you get your ground plane otherwise. If someone's got a pic of the inside of the spacer it would make much more sense, I think!

 

All I want to do is replace the antenna mount with a decent one that's not been smashed up or sprayed over and terminate it in a BNC or something sensible so we can use different radios with it - then we can just use different length antenna rods depending on the band that radio looks at. I never realised how complicated they made it! :nut:

 

Stone

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Your pic. shows the addition of a 'matching transformer' fitted. This is for VHF.

Antenna base connected via. short coax. In a Landi. a short coax between transformer and TUAAM mounted in the wing box. In the wing box a short earth lead from antenna base to the TUAAM. ALL antenna cables good mil. spec. RF coaxial cable with standard BNC connectors.

Will help more if I can but off to meeting now. See my previous post, coax fitted to antenna base via. a screw in gland/nut.

Andy.

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I'm hoping it will be a bit more obvious when my replacement antenna base turns up and I can look underneath. (on ours there's a plate in the way with a hole to let the cable through and I don't want to take the top off as it's raining!)

 

Thanks for your help so far :-)

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OK, next round of questions!

 

I now understand how the connection from antenna socket to radio is made, using the hollow-nut+washer+silicone approach, for HF antennas at least. With the VHF spacer installed it attaches to the inside of the antenna mount as if it were a HF antenna cable and presents a coaxial connector at the user end instead. Is this just standard 50 Ohm BNC? Also will it still work at higher frequencies than it was designed for? If it could do 80-140 MHz without too much attenuation it would be great for FM and airband, if it hasn't got a lowpass filter too, that is...

 

Thanks :-)

 

Stone

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Stone

 

The connector at the bottom of the cylindrical part of the VHF base is a BNC. I had a look inside one of mine a few months ago and all it contains is a small transformer with the input side connected between the BNC centre and ground (the metal body of the antenna base which is electrically connected to the vehicle body), and the output side connected between the whip socket lead and ground. The upper frequency will be determined by the performance of the transformer - I would have thought FM from 88 to 108 MHz has a good chance of working given that the normal upper limit was 76 or 79.999MHz for the various Clansman VHF sets. Air band goes up to 140MHz so may be more of a stretch.

 

If the transformer turns out to not work above 80MHz, the best thing is to remove the transformer and connect the flying lead from the antenna clamp directly to the BNC centre. This allows use of a resonant quarter wave antenna cut for the band in use - 75cm including the length of the flying lead for FM broadcast, 50cm for 2M amateur, and something in between for air band. You can always extend the whip with insulating material to 2M so it looks correct for the vehicle. The military operated over a much wider frequency range which is why they used a fixed length whip and the TUUAM antenna tuner.

 

Regards

 

Iain

 

 

OK, next round of questions!

 

..... a coaxial connector at the user end instead. Is this just standard 50 Ohm BNC? Also will it still work at higher frequencies than it was designed for? If it could do 80-140 MHz without too much attenuation it would be great for FM and airband, if it hasn't got a lowpass filter too, that is...

 

Thanks :-)

 

Stone

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  • 1 month later...

Hopefully last question :)

 

I now have a base antenna element (stripped, cleaned and repainted in Desert Sand), and a new base antenna support n.31 mk 7, along with some nice new stainless bolts to attach base to base and assembly to vehicle.

 

Which antenna elements will fit? The ones designed for manpacks have a bayonet-style pin at the bottom, but can I fit one anyway? Was hoping to use something like this(Clansman Flexible whip Antenna 0.6m 5985-99-661-6417) to keep the overall height down a bit and make it look more like a normal car radio :)

 

Cheers,

 

Stone

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Stone

 

The No.31 bases are for use with the tubular 1 metre elements that screw end to end to make 2, 3 or 4 metre vehicle whips.

 

Mine are in the shed and it is way too cold to go and look up their NSNs tonight. E-Bay item 300495998593 (which is not mine, and is not from a seller I have dealt with) has a photo. They aren't expensive - if you want a low profile solution best to use one section only.

 

The optimum length (as a quarter wave) for VHF air band is somewhere between 50 and 75cm so you probably ought to cut it back to that if air band receive is your primary use.

 

Regards

 

Iain

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Elements you will need for two meter antenna,

Bottom....5985-99-630-8456

Top........5985-99-649-8140

Middle.....5985-99-630-8455 (only used for HF, 3 or 4Mt antenna)

Noticed that my top elements are different No. than quoted in handbook, ie 5985-99-630-8457

Hope of help.

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I now have a base antenna element (stripped, cleaned and repainted in Desert Sand)

 

Erm, you didn't paint the black rubber, did you? It's meant to be unpainted. Earthing the transmitted signal and stuff.

 

And the matching transformer. It's radio kit and I doubt in service it would be painted yellow unless it happened to be on the vehicle when it got a paint job. I'll accept that in my time, the Machine was Green, so the green radio kit matched the green vehicle cam. Maybe it's changed now.

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Stone

 

The No.31 bases are for use with the tubular 1 metre elements that screw end to end to make 2, 3 or 4 metre vehicle whips.

 

Mine are in the shed and it is way too cold to go and look up their NSNs tonight. E-Bay item 300495998593 (which is not mine, and is not from a seller I have dealt with) has a photo. They aren't expensive - if you want a low profile solution best to use one section only.

 

The optimum length (as a quarter wave) for VHF air band is somewhere between 50 and 75cm so you probably ought to cut it back to that if air band receive is your primary use.

 

Regards

 

Iain

 

Clansman rods were manufactured in one-meter lengths (as opposed to four-foot Larkspur rods) which were convenient to the frequency ranges used by the various Clansman sets. Clansman VHF sets (30 - 76MHz) required quarter-wave antennae up to two meters in length. (Larkspur VHF sets, eg SR/C42 - 36-60Mhz - used eight feet of rod). The tuning units on the Clansman VHF sets expected to find 2m of rod and would automatically electronically correct the transmitted signal to make it think it was transmitting on exactly 1/4-wave. Too much rod (ie 3m, as fitted to the 321 and other HF sets) would encourage the TUAAM to try and correct the VHF signal to 1/2 wavelength and actually inhibit the transmission. It's all about wave theory. I am sure there are people more current who can explain it better than my paleantological memories.

 

Since the frequency is changed daily*, it is not practicable to cut rods to 1/4 wavelength, because if you need longer rods for tomorrow's frequency, you've got to reattach the bits you cut off.

 

If you use a mast and a dipole antenna (I just saw some great pics on a thread about vehicles in winter), Clansman Dipole antennae were marked every foot so that you could get the antenna (usually) to exactly 1/4 wavelength (rounded down to the next foot-boundary below 1/4 wavelength - see above). When using a dipole with HF, this is a lot more accurate than allowing a tuner to adjust. And the skywave element of the horizontally-polarised signal gives far, far greater range. But you cannot use a mast and a dipole on the move for obvious reasons.

 

If your set is only going to receive, the more rod you put up, the better. But it won't be militarily correct.

 

_____

* At midnight (in theory midnight Zulu time so that everybody changes frequency worldwide at the same time, but in practice at midnight local time because exercises often happen at time zone change, meaning that the scope for error and having people changing frequency on three different hour boundaries: the right time before the clock change, the right time after the clock change and the idiot who adjusts his frequency change time the wrong way because he doesn't know which way the clocks changed).

 

In the early 80s (iirc) it was decided to change frequencies at midday (in theory Zulu time) so that most units would be changing frequency in daylight, making it easier to adjust dipole etc lengths, there would be more operators about, less tired and less prone to mistake.

 

Unfortunately, during the day, combat troops were more likely to be in contact and not notice that they ought to have changed frequencies. No wonder the net just went very quiet. How embarrassing. At night, the frequency change gave the 2200 - 2359 operator something to focus on and stay awake and his successors on the dead stags the chance to verify his frequency and code changes (and remorselessly take the Mick if he'd got anything wrong) before it all kicked off at pre-dawn stand-to.

 

Iirc it only lasted one exercise season.

Edited by AlienFTM
carp speeling
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Erm, you didn't paint the black rubber, did you? It's meant to be unpainted. Earthing the transmitted signal and stuff.

That was why I replaced it, Withams got a bit slap happy when repainting the veh and it was half-desert sand and half 'can't be arsed to walk round the other side to get decent coverage'.

 

All looks spiffy now, cheers chaps :D

 

Stone

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  • 6 years later...

I came across this thread and thought I'd reopen it as I'd like to know if the OP or anyone else has had success modifying a Clansman vehicle antenna for use on the amateur 2m band. I'm unsure of the frequency response of the transformer in the standard VHF spacer ring.

 

Andy

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Andy

 

What I've done to avoid the need for a TUUAM on 4m and 6m (so should be equally valid for 2m) is to bypass the transformer in the VHF spacer ring - it's easily done to unsolder the transformer connections and link the BNC centre directly to the screw where the wire tail from the antenna base attaches. Just remember to subtract the length of the wire tail from the length of the cut down rod - I think because the wire tail is almost a one turn coil when assembled it actually has more electrical effect than its length would suggest and you will end up trimming the rod shorter still - this was noticeable at 70MHz (4m) and will be much more so at 2m, I expect.

 

Regards

 

Iain

73 de G0OZS

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Andy

 

Even at 6m and 4m it prevents a cut-to-length 1/4 wave rod from matching (I tried it with an antenna analyser to help me) which is why I bypassed the transformer.

 

I think it is only useful when used together with the TUUAM for matching electrically long elements - the standard 2m rod antenna is just about a 1/4 wave at 30MHz but 1/2 wave or longer at the top end of the band so will naturally be high impedance and inductive across most of the potential tuning range. The transformer gets the impedance down and the TUUAM tunes out the inductive reactance.

 

As a consequence if used with resonant antennas cut to length it steps down the impedance so at resonance it is well under the expected 50 ohms and does not match with the transformer present.

 

Iain

73 de G0OZS

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  • 6 months later...

Iain thank you that is most useful to know.

 

I spent many hours at WPR with a borrowed antenna analyser trying to get my Bowman base to load up on 1/4 wave whips I had calculated for 4m & 6m. I had used a SS disc in the base to house a BNC socket then a wire to the antenna socket base. But it seem the inductance of the spring, although short-circuited by the lead from BNC to antenna socket was giving weird effects.

 

As the base was on a stalk at the side of the rear body of the Wolf, much of this weirdness seemed to be due to the proximity of the rad-haz shielding.

 

So I decided to abandon that location, then bought a couple Clansman bases + matching units. (At £5 each this was a good price at the close of WPR). Now going to utilise a wing to accommodate this base. So I had been wondering whether there was an benefit in the matching unit, so obviously not thanks to your investigation.

 

Incidentally just ordered my own antenna analyser. A bit miffed as it was a "make an offer" so I did, this was considered & accepted overnight. Unfortunately the value of the pound versus the euro dropped overnight & wiped out the saving. But I noticed the offer price no longer included free postage & there was no option to see what this charge would be. So I declined the offer & bought it for the new going rate that included postage.

 

Licking my wounds that I had not bought it last night, I then got hit by an additional £25 because the Paypal exchange rate seems poorer than the rate used by e bay to give an approximate conversion figure in pounds.>:(

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Is the transformer not just a 1:1? (some time since I looked). I was told it was primarily there so that the antenna element was earthed for static prevention. I'm sure it would disappear along with everything else in the case of contact with overhead power lines.

 

Gordon

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Gordon

 

I will need to try and work out the ratio from the DC resistance next time I have one on the bench - based on my attempts at cutting elements to length I think it is around 3:1 or 4:1 - possibly an auto transformer because the impedance at resonance is far too low. It is easily bypassed by moving the connection for the antenna base pigtail from the transformer directly onto the BNC pin. The elements are still slightly shorter than expected after that, because the pigtail acts as almost a 1 turn call.

 

Regards

 

Iain

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