Hi, everybody. I'm a devoted lurker and very infrequent contributor to these discussions, and I came into this one late, so what I'm about to say may have been already covered. In which case, I apologize for my redundancy. When considering a mast-mounted VHF antenna, have you considered that a sailboat spends a good bit of its time on the water heeled over at 15 to 20 degrees? My M17 certainly did when I was sailing it on San Francisco Bay back in the '80's. When this happens, your transmitting range to stations directly abeam becomes very short indeed. If we oversimplify by visualizing your radiation pattern as a relatively flat disk emanating from the top of your mast, the radiation on your leeward side at a 15- to 20-degree heel will be hitting the water at some 70 to 100 feet from your boat, while the radiation on your windward side will be heading rather quickly towards outer space. You will have your normal transmitting range only to stations directly ahead or astern of you. On the other hand, with a hand-held unit, you can keep the antenna relatively vertical no matter what your heel angle is. So I'd think that you could avoid a fairly costly modification and obtain more consistent, if less enhanced, performance by using a supplemental antenna on your hand-held unit. That's my two cents worth. Fair winds. Les Clute ***************************************************************************** ---- Keith Diehl <kdiehl@xmission.com> wrote: ============= Nothing like a prompt reply. A larger whip antenna would help some, but an antenna tuned for the ham bands (146 MHz) probably won't have enough bandwidth to operate properly on marine frequencies (160 MHz). It certainly won't double your range. I don't think anything is going to double your range other than height. Remember that, over water, range is predominately limited by the radio line of sight horizon, which is about 4/3 the optical horizon under "typical" conditions. From memory (might be wrong!), that can be calculated as 4/3 * sqrt(height in feet / .6) If your antenna is 6' off the water, then your radio horizon is about 4 miles. If the station you're talking with has an antenna height of 50', then his horizon is 12 miles. The two of you could be about 16 miles apart and have radio light of sight. Larson used to make some large whips for portable radios that could be ordered at the proper frequency. You may want to give them a call. 1-800-ANTENNA (268-3662) ----- Original Message ----- From: "Larry E Yake" <leyake@juno.com> To: <montgomery_boats@mailman.xmission.com> Sent: Saturday, March 31, 2007 5:08 PM Subject: Re: M_Boats: VHF boost
I'm impressed, Keith. What is your opinion of the 24" replacement antenna option? Is the 2 meter band frequency OK, and would it double the range of the stock antenna as claimed? Larry
On Sat, 31 Mar 2007 12:14:37 -0600 "Keith Diehl" <kdiehl@xmission.com> writes:
I've worked as a system engineer for a major communications corporation for more years than I care to mention. Here's my analysis:
First, are we using the same antenna, but in one case mounting it at deck level and in the other case mounting it at masthead level? If so, then all we are concerned with is the additional line loss. RG8-X type cable is a manageable size and has reasonable loss - about 4 dB per 100 ft at 100 MHz. Marine radio operates at 160 MHz, so losses will be a little higher, let's call it 5 dB per 100ft.
(Don't use RG58 or Radio Shack cable or connectors. Buy quality, name brand, low-loss cable and get help if you're not sure how to install a connector. Weatherproof all connections.)
How high is your mast? 25 ft? Then loss is only 1.25 dB + maybe 0.5 dB for additional connectors to allow removing the mast. Overall, I think you'd see an improvement.
If you're using a handheld radio with a little helical whip antenna, then you'll see a big improvement. Those rubber antennas have a loss relative to a reference dipole of 6 to 10 dB. A 5/8 wave antenna will be close to the gain of a dipole reference. Even with line loss, you should see a net improvement of 3 to 7 dB.
There is a case to be made that you would like your radio to work if you are dismasted. There is also the danger of undetected corrosion or damage to the antenna and line at the masthead where inspection is difficult.
----- Original Message ----- From: "Larry E Yake" <leyake@juno.com> To: <montgomery_boats@mailman.xmission.com> Sent: Friday, March 30, 2007 9:57 PM Subject: Re: M_Boats: VHF boost <<SNIP>>
"I doubt the 3db gain -- double the radio's radiated power -- from a masthead antenna that's about 18 feet higher than the handheld's antenna will give much of an increase in range.
<<SNIP>>
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Hi Les, what you are saying is true based on the stated approximation of the monopole antenna pattern. But the pattern would need quite a large antenna to look like a disc. It will actually look much like a fat donut (see below). With a 15-20degree change in the elevation pattern you would only loose a half dB (10%)or less. As a rule of thumb the half power points on an antenna pattern are defined by the size in wavelengths as Half_power_beamwidth(radians) = wavelength / dimension_antenna So if we give the VHF antenna a dimension in the elevation plane of one half wavelength (very generous indeed) the beam width is 2 radians. This amount to 115 degrees. This means that your boat would have to be heeled at about 55 degrees before the pattern would roll off by half. In general the VHF antenna is even less directive (fatter) than that of a half wave dipole so considering it a very fat donut is not a bad approximation. Robbin Les Clute wrote:
Hi, everybody. I'm a devoted lurker and very infrequent contributor to these discussions, and I came into this one late, so what I'm about to say may have been already covered. In which case, I apologize for my redundancy. When considering a mast-mounted VHF antenna, have you considered that a sailboat spends a good bit of its time on the water heeled over at 15 to 20 degrees? My M17 certainly did when I was sailing it on San Francisco Bay back in the '80's. When this happens, your transmitting range to stations directly abeam becomes very short indeed. If we oversimplify by visualizing your radiation pattern as a relatively flat disk emanating from the top of your mast, the radiation on your leeward side at a 15- to 20-degree heel will be hitting the water at some 70 to 100 feet from your boat, while the radiation on your windward side will be heading rather quickly towards outer space. You will have your normal transmitting range only to stations directly ahead or astern of you. On the other hand, with a hand-held unit, you can keep the antenna relatively vertical no matter what your heel angle is. So I'd think that you could avoid a fairly costly modification and obtain more consistent, if less enhanced, performance by using a supplemental antenna on your hand-held unit. That's my two cents worth. Fair winds. Les Clute
*****************************************************************************
---- Keith Diehl <kdiehl@xmission.com> wrote:
============= Nothing like a prompt reply.
A larger whip antenna would help some, but an antenna tuned for the ham bands (146 MHz) probably won't have enough bandwidth to operate properly on marine frequencies (160 MHz). It certainly won't double your range. I don't think anything is going to double your range other than height.
Remember that, over water, range is predominately limited by the radio line of sight horizon, which is about 4/3 the optical horizon under "typical" conditions. From memory (might be wrong!), that can be calculated as 4/3 * sqrt(height in feet / .6) If your antenna is 6' off the water, then your radio horizon is about 4 miles. If the station you're talking with has an antenna height of 50', then his horizon is 12 miles. The two of you could be about 16 miles apart and have radio light of sight.
Larson used to make some large whips for portable radios that could be ordered at the proper frequency. You may want to give them a call. 1-800-ANTENNA (268-3662)
----- Original Message ----- From: "Larry E Yake" <leyake@juno.com> To: <montgomery_boats@mailman.xmission.com> Sent: Saturday, March 31, 2007 5:08 PM Subject: Re: M_Boats: VHF boost
I'm impressed, Keith. What is your opinion of the 24" replacement antenna option? Is the 2 meter band frequency OK, and would it double the range of the stock antenna as claimed? Larry
On Sat, 31 Mar 2007 12:14:37 -0600 "Keith Diehl" <kdiehl@xmission.com> writes:
I've worked as a system engineer for a major communications corporation for more years than I care to mention. Here's my analysis:
First, are we using the same antenna, but in one case mounting it at deck level and in the other case mounting it at masthead level? If so, then all we are concerned with is the additional line loss. RG8-X type cable is a manageable size and has reasonable loss - about 4 dB per 100 ft at 100 MHz. Marine radio operates at 160 MHz, so losses will be a little higher, let's call it 5 dB per 100ft.
(Don't use RG58 or Radio Shack cable or connectors. Buy quality, name brand, low-loss cable and get help if you're not sure how to install a connector. Weatherproof all connections.)
How high is your mast? 25 ft? Then loss is only 1.25 dB + maybe 0.5 dB for additional connectors to allow removing the mast. Overall, I think you'd see an improvement.
If you're using a handheld radio with a little helical whip antenna, then you'll see a big improvement. Those rubber antennas have a loss relative to a reference dipole of 6 to 10 dB. A 5/8 wave antenna will be close to the gain of a dipole reference. Even with line loss, you should see a net improvement of 3 to 7 dB.
There is a case to be made that you would like your radio to work if you are dismasted. There is also the danger of undetected corrosion or damage to the antenna and line at the masthead where inspection is difficult.
----- Original Message ----- From: "Larry E Yake" <leyake@juno.com> To: <montgomery_boats@mailman.xmission.com> Sent: Friday, March 30, 2007 9:57 PM Subject: Re: M_Boats: VHF boost <<SNIP>>
"I doubt the 3db gain -- double the radio's radiated power -- from a masthead antenna that's about 18
feet
higher than the handheld's antenna will give much of an increase
in
range.
<<SNIP>>
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participants (2)
-
Les Clute -
Robbin Roddewig