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Old 07-26-2017, 03:11 PM   #1
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New VHF antennas - mast vs flybridge

Over analysis paralysis here...

I have two VHF radios. New radio in the flybridge (DSC, GPS, AIS-RX) with a remote mic at the lower helm. Second, 1981 Raytheon VHF at the lower station. The TX and RX sound quality on the '81 unit is higher quality, so I may just keep it for a few more years.

The antennas need replacing. One antenna coax jacket is cracked and poor TX range. The second antenna, on 25w TX, causes the chartplotter to loose GPS fix. Assuming both antennas have corroded and failed wiring, age unknown, possibly 1981. GPS antenna is >3' from each VHF antenna.

Antennas are 8,' one mounted on each side of the flybridge. Cable runs to radios are less than 10' each.

On the mast is a LORAN antenna. The loran coax could be used to snake a new VHF line up to the mast spreader.

Here is the question, replacing two VHF radio antennas. Which option?

Option 1
Radio 1. 8' 6dB antenna fly bridge mount. Base height from water 112 inches. Less than 10' cable run.

Radio 2. 8' 6dB antenna fly bridge mount. Base height from water 112 inches.


Option 2
Radio 1. 3' 3dB antenna on mast spreader. Base height from water 202 inches. Approx 25' cable run.

Radio 2. 8' 6dB antenna fly bridge mount. Base height from water 112 inches. Less than 10' coax.


Option 2 yields two radiation options for different sea states. Use the mast radio when in bad sea state with more roll and pitch? Option 1 less install effort and lower cost.

No plans to ever be more than 25' miles from shore, coastal S. California.
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Old 07-26-2017, 03:58 PM   #2
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I like Option 2 b/c antenna height directly affects line-of-sight. But then again, quality matters, too.

Is there a way to mount a higher quality antenna on the mast spreader?
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Old 07-26-2017, 04:12 PM   #3
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I like the option where one antenna is a 10db antenna.

When you need it, nothing works better, all other things equal.

And yes, a 8 foot 6db antenna may work slightly better in heavy weather but shorter range.
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Old 07-26-2017, 07:14 PM   #4
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Option 1
The longer coax and signal loss will negate the height advantage in option 2 and adds a bridge clearance penalty
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Old 07-26-2017, 07:41 PM   #5
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Quote:
Originally Posted by psneeld View Post
I like the option where one antenna is a 10db antenna.

When you need it, nothing works better, all other things equal.

And yes, a 8 foot 6db antenna may work slightly better in heavy weather but shorter range.
I agree, but a Shakespeare 10dB antenna is 21' long. I just installed a pair of Shakespeare 5230 8db antennas that are 14' long and the range is spectacular compared to the older 8' 6db's they replaced. They are 2-section antennas and can be shipped UPS.

I ran across this little diagram when I was antenna shopping that attempts to illustrate the difference in propagation of various antenna lengths.

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Old 07-26-2017, 08:41 PM   #6
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Yep, I love theory too...

Unfortunatdly I just have to go by tons of experience.
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Old 07-29-2017, 07:06 PM   #7
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10dB antenna is 21' and appears intended for a ground station or larger commercial vessel.

21' (6.40 M) VHF Marine Band 10dB Antenna

9dB 19' antenna. I've have seen these antennas mounted on flybridge sides of many 34' and larger Trawlers around S. California. This would increase my air draft from 18.3' to 28.3' Occasional bridge issue, but I suppose I could lower as needed.

SHAKESPEARE VHF Antenna, 4018 19ft

Or, I just direct swap the two existing 6dB 8' antennas, total spend less than $200. Appears to have served well for the previous two owners, both of which have cruised greater distances.

Any other suggestions? Is there a >6dB antenna which is less than 19'?
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Old 07-29-2017, 07:34 PM   #8
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Quote:
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Any other suggestions? Is there a >6dB antenna which is less than 19'?

I doubt it. I am easily confused but it think that the antennas increase gain by using multiple antennas stacked on one another. The VHF wavelength is about 12'.
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Old 07-29-2017, 07:40 PM   #9
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Shakespeare Galaxy 5230 VHF Antenna
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Old 07-29-2017, 07:59 PM   #10
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West marine also has a 14 foot, 8dB antenna

https://www.westmarine.com/buy/shake...87?recordNum=1

Here's an 8 foot, 9dB antenna for $79 from Overtons.

Pacific Aerials P6051 VHF 8 Ultraglass Antenna
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Old 07-29-2017, 08:12 PM   #11
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My simple VHF antennae (atop the mast) is thirty-some-feet above waterline. Have good range, and better than some (many?).
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Old 07-29-2017, 08:13 PM   #12
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I also like the Shakespear Galaxy antennas. Have had them on multiple boats and had good success with them.
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Old 07-29-2017, 08:26 PM   #13
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We just put the second of our new galaxy 8'ers on today.

If you have a long run, you can use a thicker coax cable to cut down on losses.

I think the more height off the water, the better. The vhf transmissions are line of site and it won't really follow the curve of the earth.
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Old 07-29-2017, 08:37 PM   #14
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They can go faether than line of sight, but you cant bank on it.

Power can be more important (read db gain) than just height.

I want to overpower as many useless ch 16 transmissions as I can when I am screaming mayday.
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Old 07-29-2017, 08:41 PM   #15
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Thanks for all the replies.

Pacific Aerials. What is the deal with 6bBi gain vs "9dB Marine Gain." One reality, the other marketing?

P6051 - VHF 2.5M ULTRAGLASS COLINEAR ANTENNA
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Old 07-29-2017, 09:13 PM   #16
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Gain is created by redirecting the output above and below the antenna outward for more signal strength. Instead of transmitting signal up or down, it sends the signal outwards.

Db gain works on both tx and rx equally.

Height is important for distance as much as power for VHF. For HF, it does not matter much.
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Old 07-30-2017, 06:55 AM   #17
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I have been told that power makes your transmission get through over closer but weaker signals, as the radio wil pick the stronger signal as the one you hear.

Important in emergency situations and crowded boating areas.
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Old 07-30-2017, 07:55 AM   #18
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Marine radios only receive what's being transmitted in their reception area.

One more thing about dB vs power:
A 4 watt, 25 watt and 200 watt radio will transmit the same thing, but the higher power will take more power, and go a further distance, until the energy obstructed or dissipated. If you have a 4 watt walkie talkie on the dink and 25 on the trawler, the dink can hear the trawler farther away, and likely, the trawler won't hear the 4 watt talkie's response. Raw power only works when both ends have enough to illuminate the other end.

An antenna with higher gain works in two ways. The gain amplifies (without consuming any electricity) the output of your radio, as well as amplifies the reception of your radio. If your trawler has a 8-10 dB gain antenna and a 25 watt radio, it will hear better and transmit better than a 200 watt radio with a 4-6 dB gain antenna.

It is always a good idea to get the highest gain possible at the antenna. Buying a higher gain antenna is something you pay for up front once, not every time you use the radio with added power draw.

When you're talking about line of sight transmissions, height does two things. It adds range, but also adds clutter. The higher the antenna is, the more people you're likely to be hearing on the same channel. In my microwave days, we always designed the RF links to be just above the obstructions, to limit congestion. The taller you went, the more likely someone else would be polluting the frequency you're wanting.

So, taller you mount the antenna, the farther from shore you can talk back to the shore (unless they have a 400' tower on shore). Rule of thumb is every 6 feet of elevation gets you 3 miles distance. This is important when the dink is low and under powered. This means to talk to the dink from your trawler with the antenna mounted 18' off the surface, you'll get about 9 miles distance.
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Old 07-30-2017, 08:32 AM   #19
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"If you have a long run, you can use a thicker coax cable to cut down on losses."

Coax comes in multiple grades and quality levels.

The better quality with pro installed ends is worth the extra expense.

One caution with stick antennas is when laid down for bridges the ch 13 is very reduced power at the set, add it to the laid down antenna and shouting may work best.

A hand held from the dink, or a sail style antenna with a whip and loaded base coil high up might be best for inshore cruising.
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Old 07-30-2017, 09:48 AM   #20
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The laid down antenna affects the polarization of the wave.
When a stick antenna is vertical, the waves are emitted horizontally, parallel to the ground. That works best when the receiving antenna is also vertical.

It's funny to watch testosterone jockeys with their long aerials swept back following the lines of their boat and not knowing why they can't talk far on their radios.

If the bridge attendant has a walkie talkie, simply lay it horizontal when you are talking to a boat with their antennas laid back.
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