# Impedence of filament bulbs



## Vital Sparks (Sep 19, 2007)

Once when attempting to track down an intermittent aerial signal in a radio room equipment rack I noticed a box in the wiring diagram labelled "impedence matching unit". I found the item itself which was an aluminium box about 4" X 3" by 2" with a BNC connector on each end.
On opening the box I discovered inside that the only internal component was a pygmy mains bulb with the RF path passing through the filament. Does anybody know what are the RF properties of filament bulbs? (I eventually found the loose connection elsewhere).


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## Bill Greig (Jul 4, 2006)

Quite high impedance I would have thought, it would of course increase with increase in frequency.
Bill


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## trotterdotpom (Apr 29, 2005)

I always used to work out the impedance of a light bulb by using the formula 
XL = 2πfL but only if I'd run out of crosswords.

I think Lenz's Law came into it too ... "the smaller the light bulb the harder it is to see" or somat like that.

John T


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## Vital Sparks (Sep 19, 2007)

No joke, this is a genuine enquiry, the box had a part number and was on the ships drawings. The ship was the Britih Holly an ITT installation with some Marconi thrown in. The box had the usual Marconi numeric "strike off" mod version label on the lid. The box was in the incoming signal path from the aerial before it reached the distribution panel to which the receivers were connected. Since both connectors were co-axial, before opening I assumed it was possibly performing 50 to 75 ohm matching, after opening I laughed at first, then began to wonder what the RF properties of an unheated tungsten filament might be. 
Naturally the bulb was not in the parts system and so there wasn't a spare for it. No that it was in any danger of blowing.


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## Vital Sparks (Sep 19, 2007)

The aerial concerned was a 20 metre wire strung between the foremast and the focsle head which at it's base it connected to a coax cable running the length of the ship and eventually emerging in the radio room. The bulb was connected at that point. Since I was trying to answer the question "why is the forrard aerial signal intermittent?" I was interested in anything in the signal path.


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## BobClay (Dec 14, 2007)

Seems a peculiar notion to me, a mains bulb in the Rx aerial path !
Can anybody throw some light on that !! (Oh dear that's a bad pun :-0 )


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## gwzm (Nov 7, 2005)

I don't know about that particular unit but certainly in older amateur radio transceivers like the Yaesu FT101 series, there was a 12V "pea" lamp in series with the antenna internal connection to the receiver front-end. This was allegedly to protect the receiver from a catastrophic input like high voltage static or a nearby lightning strike. They did "blow" from time to time and resulted in a dead receiver.


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## GW3OQK (Jun 10, 2010)

The low power lamps were fitted as receiver input protection devices. The resistance of a lamp is low when cold, and goes higher as the lamp heats up to incandescence. Normally the receive signal passes through but a high power transmission heats up the lamp and attenuates the signal at the receiver. 

The R408 had such lamp visible on the front panel. I visited a couple of ships to receive the fault report "The lamp used to flash OK but now it has stopped and the receiver is dead." 

A couple of back-to-back 1N4148s across the receiver input is something I fit across all my receivers to limit signal levels.
73, Andrew


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## BobClay (Dec 14, 2007)

That makes sense to me. But a mains bulb ? ... do you think that might have been in there by mistake ?


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## Varley (Oct 1, 2006)

BobClay said:


> That makes sense to me. But a mains bulb ? ... do you think that might have been in there by mistake ?


I had been thinking that myself Bob and then the light came on. Before semiconductors a neon or other gas tube was used to protect the front end. Neons were available in pygmy size with standard cap and another might well have been fitted - in which case the impedance would have to be high otherwise it would be in protection mode all the time, however hot. It doesn't explain the nameplate description.

I think it likely the energy and time before a filament lamp would be hot enough to present a high resistance (and that may well be useless wrt the impedance of the front end that would have to perform the other half of the trick) would leave you protecting something black and not very good at all as a radio receiver.


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## Vital Sparks (Sep 19, 2007)

To complete the picture, the receivers on the end of this device were an IMR 5000 and a Redifon R408.


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## J. Davies (Dec 29, 2010)

SP Radio - Sailor - Series 1000 ship stations had a Receiver Protection Unit (RPU) in series with the receiver antenna inputs. They consisted of a series of low voltage bulbs as shown in the attachment. I have seen them glow bright white at times on certain frequencies, due to induced power from the transmitters. No idea how they work. It seems primitive, but they do.


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## lagerstedt (Oct 16, 2005)

Just a small note re #9 and the FT 101. I operate a FT101EE as my ham radio base station (ZL2BFO). Had a look at the manual and circuit daigram and they have the following notes. The lamp is rated at 16v 0.15amp. The notation says "Lamp Fuse - This lamp fuse protects the RF amplifier transistor from damage which maybe caused by extremely strong local signals"

Hope that helps with the debate.

Regards
Blair Lagerstedt
ZL2BFO
NZ


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## Varley (Oct 1, 2006)

I think that makes sense. The lamp (I conjecture) is simply in the transistor load. If it is overdriven the lamp simply ups the resistance to drop the current (probably not used just to burn out and open the circuit?). Where is it in the circuit of the first front end transistor?


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## BobClay (Dec 14, 2007)

I understand that. The lamp would increase it's resistance as it got hot or burn out. But for a mains bulb you're going to need some pretty heavy input before that burns out. That's why I think the mains bulb might have been put in by mistake, or because there was no spare and they needed to make the circuit.


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## Varley (Oct 1, 2006)

The neon protection would not be on the way to the front end but across aerial and earth. High resistance at low voltage. Low as soon as it strikes. Impedance in unstruck state more tricky.


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## Engine Serang (Oct 15, 2012)

This post is heavy going, was all the kit on a nuclear sub? Caution is suggested as such complexity means that it may be classified.


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## BobClay (Dec 14, 2007)

Oh WOW! .... classified information about nuclear submarines is going to bought down by a mains bulb, or a neon lamp.
Personally, I'm going to retreat to my nuclear bunker ....

[=P]


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## Engine Serang (Oct 15, 2012)

You could be right. On a visit to HMS Warspite I was amazed to find Hotpoint washing machines in the laundry, as we had at home. I had envisaged something more hi-tech than a 30, 50 and 90 degree C wash cycle. Reactor or not skiddies must be washed.


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## Varley (Oct 1, 2006)

I suppose a neon would strike at a lower voltage in a neutron flux. I suspect it would not offer much in the way of protection unless it could be seen doing so from a very long way away.


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## Varley (Oct 1, 2006)

I had not heard of antenna dischargers as such but neon stabilisers were 'in my time' (the Stabilivolt?).


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## lagerstedt (Oct 16, 2005)

Varley said:


> I think that makes sense. The lamp (I conjecture) is simply in the transistor load. If it is overdriven the lamp simply ups the resistance to drop the current (probably not used just to burn out and open the circuit?). Where is it in the circuit of the first front end transistor?


The circuit starts at the antenna input, goes through a series of resistors and capacitors which are earthed, a inline coil that has two capacitors in parrallel and a resistor to earth connected between the two capacitors. From the coil it connects to the emitter of the first stage rf amp. the circuit valves would indicate that the act as a attenuator. The circuit would not take high currents, like lighting strikes. 
Regards
Blair lagerstedt
NZ


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## Naytikos (Oct 20, 2008)

I have a feeling the installation in question is a result of the transition from thermionic to silicon technology. The purpose of the light bulb, it seems to be agreed, is/was to absorb excessive RF energy and thereby protect the receiver front ends; if the receiver were, say, an Atalanta or similar vintage type using valves/tubes and substantial tuning coils, it would probably work as intended.

As for 'impedance matching': the inductance of the filament would depend upon whether it was coiled or not and the overall form that it took within the envelope; the capacitance of the lampholder would be in series-parallel and everything would vary as the resistance changed with current flow. There's a doctorate waiting for whomever can derive a verifiable answer!

The R408, as has been mentioned, has an LES lamp directly in the aerial input circuit, which, if I recall correctly, is broadband and untuned. The lamp would burn out regularly; replacement required a piece of rubber/plastic outer insulation from a piece of co-ax. In my experience the first RF amplifier transistor (2N2369) would fail before the lamp. I used to keep a stock of either 2N2369 or 2N708s just for the purpose.

I also agree with the remark about diodes; I used to put back-to-back 1N4001 diodes across receiving aerial terminations.


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