Shortening a car aerial lead - UK Vintage Radio Repair and Restoration Discussion Forum

parlourtw73vs · 18th Oct 2017, 7:58 pm

Hello everyone, I've just got a little query which I'm hoping you can help me with. I'm fitting a new radio/cd player to my old car and the aerial lead is much longer than I need it to be. As space is a little limited behind the radio I wondered if the length of the aerial co-ax is critical to its performance or if I can shorten it to clear up a little room. Thanks in anticipation of your knowledgeable replies.

G6Tanuki · 18th Oct 2017, 8:03 pm

The length is relevant, yes: it acts as a capacitive load across the receiver's first tuned-circuit.

If you cut it down there will be less capacitance which could mean the result is outside the adjustment-range of the 'antenna trimmer' in the radio.

[similarly if you lengthen the cable you traditionally needed to introduce a series-capacitor into the coax centre-core at the radio-end to compensate for the extra capacitance the longer lead presented: this series-capacitor was usually built-into commercial radio-lead-extensions]

Why not just coil-up the excess lead and tuck it up behind the dashboard?

paulsherwin · 18th Oct 2017, 8:10 pm

In practice the length won't make much difference, but I agree it would be simpler just to coil up the excess. You may find it runs through the engine compartment, in which case you can pull it back through the firewall grommet and coil it up there.

boxdoctor · 18th Oct 2017, 8:31 pm

The admittedly few car radio aerial leads I have had dealings with have all had a spiral centre core. Presumably this small inductance is to offset the innate capacitance of the co-axial construction.
If this is the case, the length (within reasonable limits) will not greatly affect the input capacitance/inductance as the front end "sees" it.
It will probably have a negligeable effect on the tuning of the aerial circuits. Thus the aerial trimmer on the radio will be able to compensate for any slight mis-match. Tony.

Reelman · 18th Oct 2017, 8:50 pm

I don’t think many modern radios have a trimmer to adjust. The trimmer was important when MW was generally listened to, I doubt it will have any effect at VHF so is not included.

On one car I had VHF reception was fine but MW/LW was pretty poor. On investigation I discovered that the aerial plug did not quite make contact with the set. Capacitive coupling was good enough for VHF.

Peter

'LIVEWIRE?' · 18th Oct 2017, 9:56 pm

AFAIK No modern car radio, and in fact none made with digital self-seek tuning hav/has aerial trimmers. Concerning the length of the aerial cable, if the aerial is mounted at the rear of the car, an extension lead may be fitted, in which case somewher in the rear, possibly behind the trim, there will be a plug/socket connection, so a shorter extension lead could be fitted. Admittedly that isn't a five-minute job, and, as G6T and Paul Sherwin have suggested, the simpler way would be to 'lose' the excess cable by coiling it up behind the dashboard

parlourtw73vs · 18th Oct 2017, 10:19 pm

Thanks to all for your prompt assistance. The aerial is mounted on the front of the roof and the lead is passed down the windscreen pillar so that probably explains why there is so much cable behind the radio. As I originally stated, there isn't a lot of room behind it and the old unit was directly wired in, this new unit uses ISO connectors so that's another lump to accommodate and the reason for my original query. To sum up your replies, would it be safe to say that shortening the aerial lead will result in little if any deleterious effect. I usually use the radio to listen to cd's and only rarely use the radio itself and when I do it is always on FM so a minuscule loss of signal won't really worry me.

18th Oct 2017, 7:58 pm	#1
parlourtw73vs Pentode Join Date: Mar 2015 Location: Bolton, Lancashire, UK. Posts: 186	Shortening a car aerial lead Hello everyone, I've just got a little query which I'm hoping you can help me with. I'm fitting a new radio/cd player to my old car and the aerial lead is much longer than I need it to be. As space is a little limited behind the radio I wondered if the length of the aerial co-ax is critical to its performance or if I can shorten it to clear up a little room. Thanks in anticipation of your knowledgeable replies.

18th Oct 2017, 8:03 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Shortening a car aerial lead The length is relevant, yes: it acts as a capacitive load across the receiver's first tuned-circuit. If you cut it down there will be less capacitance which could mean the result is outside the adjustment-range of the 'antenna trimmer' in the radio. [similarly if you lengthen the cable you traditionally needed to introduce a series-capacitor into the coax centre-core at the radio-end to compensate for the extra capacitance the longer lead presented: this series-capacitor was usually built-into commercial radio-lead-extensions] Why not just coil-up the excess lead and tuck it up behind the dashboard?

18th Oct 2017, 8:10 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Shortening a car aerial lead In practice the length won't make much difference, but I agree it would be simpler just to coil up the excess. You may find it runs through the engine compartment, in which case you can pull it back through the firewall grommet and coil it up there.

18th Oct 2017, 8:31 pm	#4
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: Shortening a car aerial lead The admittedly few car radio aerial leads I have had dealings with have all had a spiral centre core. Presumably this small inductance is to offset the innate capacitance of the co-axial construction. If this is the case, the length (within reasonable limits) will not greatly affect the input capacitance/inductance as the front end "sees" it. It will probably have a negligeable effect on the tuning of the aerial circuits. Thus the aerial trimmer on the radio will be able to compensate for any slight mis-match. Tony.

18th Oct 2017, 8:50 pm	#5
Reelman Octode Join Date: Aug 2008 Location: Rotherham, South Yorkshire, UK. Posts: 1,723	Re: Shortening a car aerial lead I don’t think many modern radios have a trimmer to adjust. The trimmer was important when MW was generally listened to, I doubt it will have any effect at VHF so is not included. On one car I had VHF reception was fine but MW/LW was pretty poor. On investigation I discovered that the aerial plug did not quite make contact with the set. Capacitive coupling was good enough for VHF. Peter

18th Oct 2017, 9:56 pm	#6
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Shortening a car aerial lead AFAIK No modern car radio, and in fact none made with digital self-seek tuning hav/has aerial trimmers. Concerning the length of the aerial cable, if the aerial is mounted at the rear of the car, an extension lead may be fitted, in which case somewher in the rear, possibly behind the trim, there will be a plug/socket connection, so a shorter extension lead could be fitted. Admittedly that isn't a five-minute job, and, as G6T and Paul Sherwin have suggested, the simpler way would be to 'lose' the excess cable by coiling it up behind the dashboard

18th Oct 2017, 10:19 pm	#7
parlourtw73vs Pentode Join Date: Mar 2015 Location: Bolton, Lancashire, UK. Posts: 186	Re: Shortening a car aerial lead Thanks to all for your prompt assistance. The aerial is mounted on the front of the roof and the lead is passed down the windscreen pillar so that probably explains why there is so much cable behind the radio. As I originally stated, there isn't a lot of room behind it and the old unit was directly wired in, this new unit uses ISO connectors so that's another lump to accommodate and the reason for my original query. To sum up your replies, would it be safe to say that shortening the aerial lead will result in little if any deleterious effect. I usually use the radio to listen to cd's and only rarely use the radio itself and when I do it is always on FM so a minuscule loss of signal won't really worry me.