Specs of coax cables for SAT dome & GSM phones

MapisM

Well-known member
Joined
11 Mar 2002
Messages
20,360
Visit site
As per subject - Is there anyone who can advise on the best types of coax cables to use in these two applications?
Ta!

PS: by "GSM phones", I mean the cable to connect an external aerial to a GSM phone - which is actually a GSM/UMTS router, if that matters.
 

vandy

Member
Joined
4 Oct 2010
Messages
301
Location
London UK
Visit site
Being an electrical engineer I can give you the following advice:

1. For any device which is ONLY receiving singnals (i.e. a Sat TV, Radar, etc. etc.) the 75ohm Coax Cable is OK. YES THE RADAR! The Coax cable connecting your dome to the actual devcie is in fact JUST receiving the signal from the dome! so normal 75ohm cables are ok for that.

2. If the device is sending AND receiving a singnal (i.e. a VHF radio) and ONLY IF the length of the cable is going to be too high (more than 2 meters) then you better use a 50ohm so the singnal sent can be ampilified by the antenna WITHOUT noise! The ohmic losses simply cause the signal sent at nominal frequency (the frequency you want it to be sent) get weaker so other noise singnals may then be modulated as well....quite a complicated communication subject really!

For GSM phone, most coax cables are capable of 0 to 6GHz freq so as long as you use a low impedance cable (50ohm) you should be OK.

BUT what you must be careful is your antenna. The GSM antenna is different than normal atennas and those used for a VHF radio etc. as the frequency which is to be received/sent is different.

You may also consider installing an amplifier for the SAT dome.
 
Last edited:

MapisM

Well-known member
Joined
11 Mar 2002
Messages
20,360
Visit site
If the device is sending AND receiving a singnal (i.e. a VHF radio) and ONLY IF the length of the cable is going to be too high (more than 2 meters) then you better use a 50ohm so the singnal sent can be ampilified by the antenna WITHOUT noise!
Interesting stuff, thanks. Yep, length is well above 2m (from pilothouse to flybridge arch).
Am I understanding correctly your sentence above, if I say that a 50ohm cable is also fine for receiving?
Because if so, wouldn't it be simpler to route just 50ohm cables and use them either for receiving equipment or for both RX and TX, as required?
Besides, can the cables normally used for the good old terrestrial TV be used also to connect a SAT dome?
Thanks again!
 

Bilgediver

Well-known member
Joined
6 Jun 2001
Messages
8,110
Location
Scotland
Visit site
Interesting stuff, thanks. Yep, length is well above 2m (from pilothouse to flybridge arch).
Am I understanding correctly your sentence above, if I say that a 50ohm cable is also fine for receiving?
Because if so, wouldn't it be simpler to route just 50ohm cables and use them either for receiving equipment or for both RX and TX, as required?
Besides, can the cables normally used for the good old terrestrial TV be used also to connect a SAT dome?
Thanks again!

I would not use the TV cable because as was mentioned noise...low loss is a big consideration in this equation. I would use satellite cable as obtainable fron the high street satellite installation folk of even B and Q it is higher grade and low loss....If you can believe Kingfisher :D.

The mobile phone antenna seem fine with the small gauge wire they supply with car kits. If using only a dipole there is no point in long runs to gain a metre in height as the antenna gain can be lost in the cable losses. Find a local car phone installer and he may give you a length complete with the correct terminations for the antenna and output unit. They come in about 15/20 foot lengths or with external antennas. If your length is more than this then may need something better but then may get losses in the adaptors at either end :)

I just use a car licence holder antenna behind a curtain and displayiing the marina licence.:D
 

MapisM

Well-known member
Joined
11 Mar 2002
Messages
20,360
Visit site
If using only a dipole there is no point in long runs to gain a metre in height as the antenna gain can be lost in the cable losses.
That's a good point. I did consider it, but I guess that I'll never know for sure unless I try.
The point is, in some of my favourite overnighting bays, the mobile signal is very poor.
I was hoping that a good antenna, mounted on the top of the flybridge (hence with a difference of 3m or so, plus the open air position), should more than compensate the cable losses.
This is the antenna I was considering (good for a 6db gain, apparently), but I'm open to suggestions.
My initial question was only related to wiring because at the moment, within the framework of other works I'm doing onboard, it makes sense to snake the cable in advance.
 

Bilgediver

Well-known member
Joined
6 Jun 2001
Messages
8,110
Location
Scotland
Visit site
That's a good point. I did consider it, but I guess that I'll never know for sure unless I try.
The point is, in some of my favourite overnighting bays, the mobile signal is very poor.
I was hoping that a good antenna, mounted on the top of the flybridge (hence with a difference of 3m or so, plus the open air position), should more than compensate the cable losses.
This is the antenna I was considering (good for a 6db gain, apparently), but I'm open to suggestions.
My initial question was only related to wiring because at the moment, within the framework of other works I'm doing onboard, it makes sense to snake the cable in advance.

It looks a good substantial antenna and probably has a type N connector . You might have to make up an adaptor lead or something maybe utilising 3 metres of car antenna lead .

Check your yellow pages for an installer of cell phones in cars and he might have a spare lead which would come with standard fittings however you would need to adapt the antenna end by fitting a BNC crimp type which would leave you to get a type N to BNC male adaptor from Maplins The coax is about 4mm and comes in about 5 metre lengths.

By the way......MOst modern cell phones when used in a car kit do not have provision for an external antenna! They use the built in antenna in the phone?????? Now if you are a dinasaur like me and use a Nokia 6310i then no problem as the car kit puts out extra welly and there is a connector for an antenna. However there is one other connection you may not know about.... Nokia do a handset and bracket on a long curly lead. This plugs into the Data socket instead of your fax machine:)

This hand set is much easier to use than the car mike and the whole marina doesn t hear both sides of your phone call.

The phone rest has a magnet which activates a switch in te hand set so replacing it cuts the call. I find it handy when sailing as I can take calls in the cockpit without having to shout at a mike in the cabin.
 
Last edited:

MapisM

Well-known member
Joined
11 Mar 2002
Messages
20,360
Visit site
By the way......MOst modern cell phones when used in a car kit do not have provision for an external antenna!
Yep, I know. But as I mentioned in the OP, it's not actually a mobile phone that I'm thinking to use with the antenna, it's this router.
It works as a wireless hotspot, servicing any onboard PC. And it does have an external antenna connector.
Great piece of hardware btw, highly recommended.
And if I can have a reliable data connection, I don't care if the handy mobiles are out of coverage.
It's even better, at times... :)
 

Bilgediver

Well-known member
Joined
6 Jun 2001
Messages
8,110
Location
Scotland
Visit site
Yep, I know. But as I mentioned in the OP, it's not actually a mobile phone that I'm thinking to use with the antenna, it's this router.
It works as a wireless hotspot, servicing any onboard PC. And it does have an external antenna connector.
Great piece of hardware btw, highly recommended.
And if I can have a reliable data connection, I don't care if the handy mobiles are out of coverage.
It's even better, at times... :)

Nice bit of kit....Bit smarter than my Nokia car kit which can just about handle GPRS :D It will have a proprietry connection of some sort however I can t find it described on the site.. Mine has TNC .
 
Last edited:
Top