'Coiled' TV antenna coax

[JohnW2]

Hi folks,

A friend recently asked me a question to which I was not certain of the answer (without 'doing the experiment'!), so I thought I should look to some of the brains here!

I don't know the background/reason for his question, but he asked whether if part of the length of a coax cable fed from a TV antenna were 'coiled up' (e.g. wound on a reel/drum) that would create an inductor ('choke') that would impair, maybe even totally block, transmission of the TV signal along the cable.

Without much thought, my initial (sort-of 'intuitive') inclination was to answer something like 'probably not much', but I then realised that I might struggle with a follow-up question of 'why not?', so decided to keep my mouth shut!

If one of the conductors of a cable were 'coiled' on its own, that presumably would create a 'choke' which could seriously impair transmission of a UHF signal, but something seems to be telling my intuition that this would be much less of an issue (f any) if both conductors were 'coiled together'.

What is the actual answer?

Kind Regards, John

 

[RandomGrinch]

A while ago, we did run a similar "Transmission Line" experiment - a 100kHz signal generator feeds 100M of RG-58, with a switchable resistance load on the other end.

Although I can't answer the why, I can say that (within our experimental parameters), there was no discernible difference (Edit: I should say, the differences were neglible compared to the other experimental variables) to the signal, whether the coil remained on the drum, or if it were partly uncoiled.
The predominant changes were down to the switching of resistances - it was the impedance matching (and mismatching) that was being investigated at different frequencies.

Would the coax design partly cancel out the effects of capacitance and inductance anyway, for the frequencies it was designed for (hence the desire for an impedance matched system)?

One of the dominating factors in the inductance equation is the number of turns on the coil - which on a drum of RG-58, might not be that many (relatively)?

I'll have to dig out the lab script...

 

[bernardgreen]

I would not venture an answer other than an undocumented recollection that the SWR ( Standing Wave Ratio ) of a transmitting antenna on an extending mast varied depending on how far the mast was extended, Was it the coiled feeder cable or the varying distance between the antenna and the roof of the vehicle or both.

A mast like this

 

[JohnW2]

A while ago, we did run a similar "Transmission Line" experiment - a 100kHz signal generator feeds 100M of RG-58, with a switchable resistance load on the other end. .... Although I can't answer the why, I can say that (within our experimental parameters), there was no discernible difference ..... to the signal, whether the coil remained on the drum, or if it were partly uncoiled.

Thanks. So my 'intuition' may not have been totally off-track

However, do you really mean 100 kHz? If so that's not far short of 4 orders of magnitude lower than the TV frequencies we're talking about, a difference which might be very significant!

Would the coax design partly cancel out the effects of capacitance and inductance anyway, for the frequencies it was designed for (hence the desire for an impedance matched system)? ... One of the dominating factors in the inductance equation is the number of turns on the coil - which on a drum of RG-58, might not be that many (relatively)?

I don't think there is any doubt that if we were talking of just a single conductor, then just a small number of turns would be enough for the reactance to be very significant at, say, 600 MHz. What I'm very uncertain about is the question of what happens when both conductors are closely related and identically coiled.

Kind Regards, John

 

[JohnW2]

I would not venture an answer other than an undocumented recollection that the SWR ( Standing Wave Ratio ) of a transmitting antenna on an extending mast varied depending on how far the mast was extended, Was it the coiled feeder cable or the varying distance between the antenna and the roof of the vehicle or both.

As I understand it, SWR will only vary if the relationship between the impedance of the load/termination and the 'characteristic impedance' of the feeder changes - and I wouldn't have thought (although certainly may be wrong!) that coiling would affect characteristic impedance (appreciably if at all) - that (I thought!) being primarily dependent upon the dimensions of the inner and outer conductors and the nature of the material between them.

As for the question I was actually asked, I may try the experiment. I have a TV and an antenna, and I imagine I could find a small drum of some sort of coax lying around somewhere!

Kind Regards, John

 

[JohnW2]

Coiling the coax will not affect the balanced signal in the coax.

Thanks. Again, that corresponds with my intuition. However, can you help with the 'why (not)?' question?

Kind Regards, John

 

[RandomGrinch]

However, do you really mean 100 kHz? If so that's not far short of 4 orders of magnitude lower than the TV frequencies we're talking about, a difference which might be very significant!

That's why I said it was a similar experiment
I was wrong though, we used a Thandar TG105 5MHz pulse generator, still way off of TV frequencies, but we tend not to let students play around with anything much higher.

 

[JohnW2]

That's why I said it was a similar experiment I was wrong though, we used a Thandar TG105 5MHz pulse generator, still way off of TV frequencies ....

Fair enough but, as you say, that's still a couple of orders of magnitude off TV frequencies. If it were a single conductor, a tiny amount of inductance would presumably have enough reactance to essentially 'block' a ~500 MHz TV signal?

, but we tend not to let students play around with anything much higher.

As a matter of interest, why not ?! When I was a teenager, I was sometimes 'playing' with 1300 MHz signals

If this experiment was being undertaken by/for students, there was presumably an expectation that students would ask the "why?" question in relation to the results o9f the experiment, so I wonder what answer to such a question was 'up your sleeve'?!

Kind Regards, John

 

[flameport]

Was it the coiled feeder cable or the varying distance between the antenna and the roof of the vehicle or both.

Height or more correctly distance between the antenna and the ground would have a very significant effect on the impedance - so much so that any effect from a coiled cable would be undetectable.

 

[JohnW2]

Height or more correctly distance between the antenna and the ground would have a very significant effect on the impedance - so much so that any effect from a coiled cable would be undetectable.

I'm sure that's true, but the ('my', or my friend's) question is whether or not, for a given distance between the antenna and the ground. the difference between coiled and non-coiled cable would "be detectable"? Do you have a view on that?

Kind Regards, John