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Old 29th Sep 2014, 9:48 pm   #1
high_vacuum_house
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Default Murphy B40 aerial configuration

Good evening,
I have been trying out a Murphy B40d tonight and was wandering what the correct configuration of the aerial. The aerial input on the B40d is a balanced transformer, though I have a long wire aerial in the garden. Can I connect one end of the transformer to ground and the other to the long wire aerial or do I need something between them like a random wire aerial tuner unit.
Many thanks,
Christopher Capener
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Old 29th Sep 2014, 10:35 pm   #2
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Smile Re: Murphy B40 aerial configuration

I have seen a 10-160 metre random wire ATU for sale nearby. What effects would this device have on radio reception on a Murphy B40 or an RA17 if added between a single ended long wire aerial and a receiver.
Many thanks,
Christopher Capener
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Old 29th Sep 2014, 10:53 pm   #3
g4aaw pete
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Default Re: Murphy B40 aerial configuration

Hello Chris

A random wire will have a random feed impedance.
A matching unit will be able to tailor this impedance to suit the balanced input on your receiver.

However, as seen below, you could add an extra connection to the RF stage on the B40d. The B40a, b & c had this facility ( terminal D), which would perhaps suit an unturned aerial. These circuits are taken from the AP 57140 series manual.

Regards
Pete
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Old 29th Sep 2014, 11:01 pm   #4
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Smile Re: Murphy B40 aerial configuration

Many thanks for your reply,
So I could connect the aerial to the left side of C104 between the aerial coupling transformer.
P.s do you know where SW101 is and it's function
Many thanks again,
Christopher Capener
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Old 29th Sep 2014, 11:36 pm   #5
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Default Re: Murphy B40 aerial configuration

A value of about 10pF connected to the left of C104 should give you a high impedance input.

SW101 shown is a distorted view of SWM, which would appear to be a muting switch.
This is intended to short the aerial input terminals to ground when a transmitter is used on the same frequency as that being received.

Last edited by g4aaw pete; 29th Sep 2014 at 11:38 pm. Reason: clarification
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Old 29th Sep 2014, 11:46 pm   #6
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Smile Re: Murphy B40 aerial configuration

Many thanks!
I shall have a play a round with some low value C's and see what happens !!
Many thanks again. I am a little clueless when it comes to understanding correct radio aerial configurations and matching!!
Christopher Capener
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Old 30th Sep 2014, 11:48 am   #7
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Default Re: Murphy B40 aerial configuration

I use the original plessey connector combination with rg58 milc specs coax. This connected to the low impedance connections. The coax is on the other side connected to a DIY 1:9 un/un impedance matching to a long wire (random length). Better is connecting the low impedance inlet to a antenna tuner.... and this connected to a dipole long wire verttical whatever you prefer.
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Old 23rd Oct 2014, 3:01 pm   #8
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Default Re: Murphy B40 aerial configuration

Have found this explanation of the B40D input circuit, explains how the aerial and receivers were configured for common aerial working.

The switch SW101 is operated by a cam on the on the waveband turret assembly,
it will be in the normally open position unless the turret is being moved from one band to another when it closes so allowing the aerial to stay connected to receivers further down the line. Note the terminating resistor. The switch is a quick acting microswitch, so will have a minimum of dead space while changing state.
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Old 29th Oct 2014, 5:46 pm   #9
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Default Re: Murphy B40 aerial configuration

Back in the early 1950s I had an R107, it had two antenna inputs. One was, a single, presumably, high impedance terminal in reference to earth/ground (the metal case/chassis was grounded), it was loosely coupled to the first tuned circuit. The other was low impedance inductively coupled to the first tuned circuit. This was for 'balanced' or co-axial antenna input. The reason for mentioning all this is because it didn't seem to make much difference, at least over the 1.2Mcs to 17.5Mcsrange of the Rx. It seemed to have more than adequate sensitivity.
I've often wondered since if a very simple impedance matching device might work? For example wind a very simple untuned coil (100 turns say), connect the 'top' end to the aerial, bottom to earth/ground. Connect your radio to a) At top of coil for those occasions when the random length of your antenna is high. To b) Say 10 to 20 percent of the way up the coil, for those occasions when the random length antenna is possibly resonant and therefore low? Try different combinations. Just an idea anyway.
BTW be careful about 'tapping onto' any internal receiver circuit lest we could put the input circuit 'off tune'. Regards VO1TMM
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Old 30th Oct 2014, 12:40 pm   #10
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Default Re: Murphy B40 aerial configuration

Quote:
Originally Posted by Tyso_Bl View Post
Have found this explanation of the B40D input circuit, explains how the aerial and receivers were configured for common aerial working. The switch SW101 is operated by a cam on the on the waveband turret assembly, it will be in the normally open position unless the turret is being moved from one band to another when it closes so allowing the aerial to stay connected to receivers further down the line. Note the terminating resistor. The switch is a quick acting microswitch, so will have a minimum of dead space while changing state.
Can you give me the source of this info. It looks like a B.R. Handbook. I have the BR333.

best regards Qwenix
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Old 30th Oct 2014, 1:45 pm   #11
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Default Re: Murphy B40 aerial configuration

@Quenix

From memory not was not BR333, i think it came from HMS Collinwood museum web site, anyway here it is, it seems to be an extract from another document, but I can't work out which.
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Old 30th Oct 2014, 2:40 pm   #12
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Default Re: Murphy B40 aerial configuration

The B40 was a naval receiver, and as such was used in one of the most hostile environments a receiver could find itself in.

They were interested in multiple operators working different links as well as co-sited transmitters working on other frequencies. It pre-dates the use of multicouplers - active amplifiers with multiple outputs to drive lots of receivers. Such a machine would have to be quite something to work in this environment. It was done, though later, and the things used lots of power.

So the B40 was designed for a scheme where signal was passed serially through the tuned stages of several receivers. Each tuned stage would act as a short at other frequencies and pass the signals on. It would only drop voltage within its own bandpass width.

You don't have a battleship, you don't want to use an array of B40s all at once and your floor wouldn't support them anyway. So how do you couple in a random antenna?

You can put the antenna onto terminal C, then strap terminals A and B to ground the other end of the primary. THis should work well, but will be down a bit on signal strength due to mis-match at some frequencies. You might not even notice on the lower bands The mismatch will reduce QRM just as much.

You can add the circuitry for terminal D and use it. As above, it will work well and have similar limitations.

Both of these methods risk the antenna off-tuning the input tuned circuit and you'll lose a bit. Some sets more intended for random antennae had antenna tuning controls which were just trimmers to pull the tuned circuit a little to allow you to compensate.

In lieu of this, a little outboard antenna tuner would help a bit at some frequencies. Again, it would be more noticeable on the higher bands, and they are open and working well nowadays.

David
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Old 30th Oct 2014, 9:57 pm   #13
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Default Re: Murphy B40 aerial configuration

Well I have about 10 B40 b41 62B and more running and these rigs are all connected via switches to wether a un/un balun DIY 1:9 or a heathkit SA2040 or viewstar vs300A aerial tunit unit. This works very well with me.

The CAW subject was discussed in another topic. Ive been looking (searching my way through) around the internetsite of collingwood or radar and communications museum but never found the explanation. I knew a bit about CAW but never got around finding a circuit + explanation so thanks for sharing this matter.

best regards Qwenix
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