VHF aerial and radio

adelaidem

New member
Joined
1 Mar 2005
Messages
296
Visit site
Could anyone give me some advice on installing a VHF aerial??
1) navman VHF 7000 can that recieve the same results running through a VHF/UHF/and FM, TV aerial ?
2) ok if possible, where would be the best place to fit the antenna,spreaders,or mast top or will i gain anything by leaving it around deck level.
3) oh and can the Navman VHF 7000 be flush mounted?
hope someone can help me would be much appreciated......
 

Vulcan

New member
Joined
22 May 2004
Messages
81
Location
France
Visit site
VHF Radio and Aerial need to be matched, any old aerial will not do for good performance. Aerial should be as high as possible and vertical.
 

Benbow

New member
Joined
11 Jan 2004
Messages
1,202
Visit site
Don't know anything about that radio, but
1. you must use a proper marine VHF aerial or you will be very sorry - you could wreck the radio.
2. You will loose a lot of range by leaving it at deck height. The higher the better.
3. no idea. Well actually a quick google shows, "Flush and bracket mount options and dust cover included as standard."
 

adelaidem

New member
Joined
1 Mar 2005
Messages
296
Visit site
ok so sounds like i need to install a TV antenna and a VHF antenna on the mast top just thought might be able to split the one antenna to VHF radio,TV and AM/FM stereo
 

Benbow

New member
Joined
11 Jan 2004
Messages
1,202
Visit site
That's a different question!

You can get splitters that allow you to use your VHF aerial for your AM/FM stereo; I believe they have been discussed here before. I would not recommend them however, personally I would give priority to a simple and optimal VHF installation.
 

Salty John

Active member
Joined
6 Sep 2004
Messages
4,563
Location
UK
www.saltyjohn.co.uk
VHF aerial will use 50ohm coax, not 75ohm for TV, so you can't mix them. VHF masthead aerial should be a 1/2 wavelength 3dB gain, shunt fed unit with less than 1.2:1 SWR with frequency range 156 - 163MHz. Whip length will be 34 inches. Don't worry about what this all means, most good quality aerials comply. The 3dB gain is important for masthead application because the signal shape is more like a fat donut and still picks up a signal when heeled, whereas a 6dB or 9dB gain aerial has a flatter donut which tends to point at the sky or the water when heeled, losing effectiveness.
The aerial will have a connector, an SO239, in the base to which you will run the coax. To the coax you will have fitted a PL259 plug which fits this socket. Use good quality connectors and solder. You can get an aerial with a cable factory-crimped to it, but then you can't easily remove the aerial when you lay up the boat. Also the cable will be RG58 which is actually for short runs, usually interconnecting deck level equipment. Also the crimped connection can whick up moisture. Usually such aerials are supplied with only a short length of cable, maybe 4 meters, indicating their intended use on smaller power boats. For runs up to about 15 meters use RG8X, for longer runs use RG8. You can get RG213 cable but it is not worth the hassle of more difficult connections compared to performance, so avoid it even though it is the best.
Use a good quality cable with UV resistant cover.
Hope this information helps,
Cheers,
John
 

adelaidem

New member
Joined
1 Mar 2005
Messages
296
Visit site
well that all sounds like good advice but now im thinking if i mast mount my VHS antenna and if i was to lose my rig to put out an emergency PAN PAN PAN, i would have to have an emergencey antenna so really im thinking how much will i gain going up the mast as to a deck mount antenna?? The joys of refitting a yacht with out a rig!!!
 

Benbow

New member
Joined
11 Jan 2004
Messages
1,202
Visit site
Well yes but what if you have an emergency without loosing your mast! Why halve the range of the radio? You should have an emergency aerial stashed away somewhere that you can bring out if you loose the mast.

(make sure you can access the back of the radio to connect the spare aerial !)

PS I would strongly advise not using a deck plug. If you need to bring the cable through the deck use a gland through which you can pass the connector. You thus have an unbroken cable from radio to masthead.
 

adelaidem

New member
Joined
1 Mar 2005
Messages
296
Visit site
ok so VHF aerial on mast top 40ft RG8 cable and all the plugs to suit, a spare emergency VHF aerial ,and now im thinking TV/UHF/AM/FM disk type antenna on the 1st or 2nd set of spreaders hows that sound?????
 

Salty John

Active member
Joined
6 Sep 2004
Messages
4,563
Location
UK
www.saltyjohn.co.uk
Worse case would be ship-to-ship communication where your antenna at, say, 10' above water would see the horizon 4 miles away. You can add about 20% for the ability of the signal to bend over the horizon slightly, and a tiny bit for high power transmission, so you are signalling to 5 miles. The receiving vessel, set up the same way, has the same performance, so 10 miles is your combined range.
Your range to a Coast Guard shore radio station would be better, maybe 15 miles, because such stations are usually well elevated and have good gear.
You could probably use a 6dB gain antenna at deck level, but it still would seriously underperform the masthead unit.
For an emergency antenna you can use a piece of wire (coat hanger type), 18 inches long, pushed into the antenna socket at the back of the radio. Don't use such an arrangement for extended use on high power, and try not to short it to the radio socket outer surface as this could damage the radio.
 

Howardnp

New member
Joined
21 Jul 2004
Messages
162
Location
Worcester
Visit site
As an emergency, instead of a coathanger just split the co ax, 18 inches of the core up a broom handle, 18 of screen down the handle and done.

Mind you splitting the co ax might take a while... back to the coat hanger!!
 

ongolo

New member
Joined
5 Aug 2003
Messages
487
Visit site
In a PBO about 3 ? years ago, was a description of how to make a VHF aerial, I made two so far, they work like a charm.

ongolo
 

Vara

Active member
Joined
20 Feb 2004
Messages
7,015
Location
Canterbury/Dover
Visit site
Getting away from the boaty side and onto the quayside.
Can I advise against siting the TV aerial anywhere permanently,after having a hard installed one which either worked or not,I now have the aerial on a wander lead so that I can move it about to optimise reception.
Aerials are mainly science but theres a bit of art as well,and on a number of occasions I have found that higher is not necessarily better,or that moving the aerial a couple of feet has dramatically improved picture.
 
Top