Replacing vhf cable
- Thread starter SHADS
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Most VHF antennas are supplied with RG58U or equivalent cable. This is only 0.2" in diameter and is easy (? huh) to pass down a mast duct.
A 50' length of RG58U will attenuate VHF radio signals by about 30% as opposed to 16% for the low loss types which are 0.4" diameter - so Don S/V is right.
However VHF being 'Line of Sight', and with a masthead antenna, your VHF range is more likely to be limited by your mast height than the signal loss in the antenna cable.
A 50' length of RG58U will attenuate VHF radio signals by about 30% as opposed to 16% for the low loss types which are 0.4" diameter - so Don S/V is right.
However VHF being 'Line of Sight', and with a masthead antenna, your VHF range is more likely to be limited by your mast height than the signal loss in the antenna cable.
Match the impedance of your transmitter
Check you VHF radio manual for the output impedance of the radio antenna jack. Most are 50 ohm and require 50 ohm cables and antenna to operate with any efficency. If you try to use TV coax (75 ohm) cable it will not work too well. It could result in power being reflected back to the radio and frying it.
Don's recommendations are both 50 ohm. if you have to use a connector in the middle for when you unstep the mast make sure it is 50 ohm also. The connectors come in both 50 and 75 ohm versions and there is no way to tell the differance once they are "out of the bag"
Check you VHF radio manual for the output impedance of the radio antenna jack. Most are 50 ohm and require 50 ohm cables and antenna to operate with any efficency. If you try to use TV coax (75 ohm) cable it will not work too well. It could result in power being reflected back to the radio and frying it.
Don's recommendations are both 50 ohm. if you have to use a connector in the middle for when you unstep the mast make sure it is 50 ohm also. The connectors come in both 50 and 75 ohm versions and there is no way to tell the differance once they are "out of the bag"
On this short run I'd just use a good RG-58 (Belden 8214 for example).
Just make sure it has a dense outer sheild.
As Donalex states, it's smaller, easier to fish. Less weight aloft. More flexible where you need a pigtail.
As to the much touted half your power (3db), an old friend who was an RF design engineer once told me that a 3db rise is good for about 14% more range. So if you can talk 10 miles, you'll get 11.4, if 20 then 22.8, and so on. My experience over the years has borne this to be a good rule of thumb.
Just make sure it has a dense outer sheild.
As Donalex states, it's smaller, easier to fish. Less weight aloft. More flexible where you need a pigtail.
As to the much touted half your power (3db), an old friend who was an RF design engineer once told me that a 3db rise is good for about 14% more range. So if you can talk 10 miles, you'll get 11.4, if 20 then 22.8, and so on. My experience over the years has borne this to be a good rule of thumb.
Don't know where the distance vs. half power rationale comes from as they are mathematically unrelated. What a 3dB loss can do definitively is make the difference between being heard calling Mayday and not being heard, especially if the signal is marginal to begin with. FM intelligibility is not varying degrees or loudness; rather, it is either in or out.
If your preference is to save a few pounds of coax weight or a few dollar in cost for cheaper coax at the expense of being heard, ... a choice everyone makes for themselves but I'd personally like to know before I let my family get on your boat.
If your preference is to save a few pounds of coax weight or a few dollar in cost for cheaper coax at the expense of being heard, ... a choice everyone makes for themselves but I'd personally like to know before I let my family get on your boat.
With the length of cable on a sail boat, just about any 50 ohm cable AND connectors will do just fine. Very important to use 50 ohm connectors as the impedance mismatch with 75 ohm connectors will give you a bad VSWR (voltage standing wave ratio). Mismatches are something akin to seeing waves hit a quaywall. The reflected waves return toward the source and negatively affect the desired waves. If the VSWR is high enough, it will burn out your transmitter power section in the radio. RG 58 is fine. RG 213 will be marginally (but probably not noticably) better.
Don, I'm just applying what experience has shown me the last 30 years of working on LMR, public safety, and broadcast radio.
In the real world, a 3db increase (with no other changes) generally gives you marginal increase in range, if you were noisey, you're a bit quieter. Yes, if unreadable, maybe gets you over the threshold. Of course if you have that bad of a signal boat roll is going to come into play. Even with just a quarter wave antenna.
It's a 26' boat. The mast should be about that length. Not a long run.
Now if we were talking about 50' @ 450 Mc, that's a different thing.
In the real world, a 3db increase (with no other changes) generally gives you marginal increase in range, if you were noisey, you're a bit quieter. Yes, if unreadable, maybe gets you over the threshold. Of course if you have that bad of a signal boat roll is going to come into play. Even with just a quarter wave antenna.
It's a 26' boat. The mast should be about that length. Not a long run.
Now if we were talking about 50' @ 450 Mc, that's a different thing.
If by R6 you mean RG-6, that is 75 ohm cable. It is the same as RG-59 but with less loss for longer runs. Great for CCTV - bad for radios.
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