VHF Antenna Suggestions?

[seabob4]

Galaxy 5225XT...

Now where have I seen this before?...

[MarshMarlowe196]

The 8 footer will have negligible improvement.

Using a line of sight calculator  (I'm sure it rounds off to the nearest whole mile.)


-assuming a shore radio with a 100' antenna.  (Port Richey TowBoat antenna is 1000' high)
-assuming from my photo 3' boat antenna maybe 8' off the water.

Line of sight distance = 18 miles.

-assuming an 8' antenna on the gunnel = 10' off the water

Line of sight distance = 18 miles.

You have to put in a 12' height to get the calculator to change to 19 miles.

http://www.qsl.net/kd4sai/distance.html

This is my thinking too.

Honestly guys, I think I've been in 6' swells once in my boat, and I was well within sight of the the shore.  I almost never venture out more than 5 miles from shore in my 19-6.  Thanks to my GF's father, an offshore trip is planned and we are comfy safely cozy on a very young 26' Boston Whaler W/A Conquest with twins and a 12' VHF antenna.

I understand the safety aspect of the bigger antenna, but with the kind of fishing I do 98% of the time, it would be almost the same as having an 8' Antenna on a bass boat   .

[gran398]

You'll be well pleased with the Shakespeare squatty, in either format.

[John Jones]

I understand the safety aspect of the bigger antenna, but with the kind of fishing I do 98% of the time, it would be almost the same as having an 8' Antenna on a bass boat   .

 

[DOCREED]

If you are going to go big, go all the way.  10db

http://www.digitalantenna.com/prods/vhfantenna_16.html

10dB is too much for these boats and will cause the signal to fade in and out as the boat rocks


odb gani is like a circle around the antenna

as the gain i increased, the top and bottom of the circle get squashed so you can hear and transmit further up to a point

[wingtime]

Docreed is right 9 or 10 db is for a large ship or a land mount antenna that does not roll.  With that high of a gain the signal radiates from the antenna in almost flat a disc shape.  So when the boat rocks a few degrees your signal is now transmitting into the air and into the water not at the horizon.  A 3db radiates from the antenna in a torus or doughnut shape so when the boat rocks a portion of the signal is still transmitting toward the horizon.  This makes the 3db ideal for mounting on top of a sail boat mast or a small boat that rocks.  6db is the highest gain you would want for our boats since it is a little flatter pattern but will still radiate signal towards the horizon if the boat rolls a bit.  Keep in mind the db gain is not actually amplifying the signal... that is impossible... it just reshapes the signal pattern so in certain locations it would seem like the signal is amplified.

[John Jones]

If you are going to go big, go all the way.  10db

http://www.digitalantenna.com/prods/vhfantenna_16.html

This was a joke...

**************************************

I understand antenna radiation patterns and already gave my version of the effects of gain.

Higher gain does concentrate the radiation pattern for slightly more range but the tighter radiation pattern makes the radio fade in and out when the boat rolls.  Think of a donut laying on the ground and that's what the radiation pattern looks like from a vertically polarized antenna.  Now mash the donut flat.  Yes it spreads out in area covered but now the vertical coverage is concentrated in a flatter pattern.  Tilt it a little like the boat rolling and you overshoot or undershoot your target.  Like everything else one has to pick the least of the evils for their own purpose.  (antennas and radio communications were my first career choice many moons ago)

[RickK]

Beamwidth - the higher the db the narrower the beamwidth
----------------------------------------------------------
Range: http://reviews.ebay.com/VHF-Marine-Radio-Antenna-Buying-Guide_W0QQugidZ10000000000015940
Square Root of Height Above Water (in feet) times 1.42 equals Range in Miles

Example: Highest point of your boat is 6 feet above water. You attach your 3-foot antenna at that point. The antenna is now 9 feet above water. The square root of 9 (which is 3) times 1.42 equals 4.26 miles

[seabob4]

Rick,
That was a good read...

[gran398]

If you are going to go big, go all the way.  10db

http://www.digitalantenna.com/prods/vhfantenna_16.html

This was a joke...

**************************************

I understand antenna radiation patterns and already gave my version of the effects of gain.

Higher gain does concentrate the radiation pattern for slightly more range but the tighter radiation pattern makes the radio fade in and out when the boat rolls.  Think of a donut laying on the ground and that's what the radiation pattern looks like from a vertically polarized antenna.  Now mash the donut flat.  Yes it spreads out in area covered but now the vertical coverage is concentrated in a flatter pattern.  Tilt it a little like the boat rolling and you overshoot or undershoot your target.  Like everything else one has to pick the least of the evils for their own purpose.  (antennas and radio communications were my first career choice many moons ago)

I got it too  

[gran398]

Beamwidth - the higher the db the narrower the beamwidth
----------------------------------------------------------
Range: http://reviews.ebay.com/VHF-Marine-Radio-Antenna-Buying-Guide_W0QQugidZ10000000000015940
Square Root of Height Above Water (in feet) times 1.42 equals Range in Miles

Example: Highest point of your boat is 6 feet above water. You attach your 3-foot antenna at that point. The antenna is now 9 feet above water. The square root of 9 (which is 3) times 1.42 equals 4.26 miles

Rick, good job. Am making the assumption that the quoted line of sight transmission of 4.26 miles is based upon the receiving station's antenna height being the same, i.e. 9 feet above sea-level?

Thanks.

[flounderpounder225]

Kind of related to this discussion, I noticed the other day when installing my new Icom, the cable coming from my antenna, about 6" from the base of the antenna, the outer coating cracked, probably from the radius bend created from raising and lowering the antenna, you can see the shield of the coax, everything is intact, but there of course is an opening in the skin of the cable, which would allow moisture, corrosion etc... thinking of sealing with some west marine liquid electric tape, the black gooey stuff?? Opinions on reception/transmitting with a compromised jacket??

[gran398]

If its just the outer vinyl/rubber compromised, and not the interior metal weave, I'd say you're good. Repair as stated...you are again ready to 'splain how the biggun' got away....

[RickK]

Rick, good job. Am making the assumption that the quoted line of sight transmission of 4.26 miles is based upon the receiving station's antenna height being the same, i.e. 9 feet above sea-level?

Thanks.

The article explains the basics though, which is enough for most of us - it can get very complicated as you try to design a point to point link. I am no expert but I do work in the RF industry and am learning more each day.
Beamwidth - to keep this very simplistic - on an omni antenna think of the radiated signal as a donut of radio signal around the tip (as mentioned already here) - 360 degrees around.   But to get gain think of the signal being squashed into a sort of "V" (remember I said simplistic) that the donut now radiates in.  If two antennas can "see" each others "V", or the "V"s cross, then life is good because they lie within each others beamwidth.

If you read just below the distance example in the article it explains that you have to do the calcs for the other end also.
If we do the calcs on the 1000ft antenna mentioned above:  The sq rt of 1000 is approx 31 X 1.42 = approx 45 miles.
I would hope that the high antenna was one that has a wide vertical beamwidth (like a 3db) so it would receive a signal from a much lower height antenna, even if someone had increased the power using a higher gain antenna, but ended up narrowing it's beamwidth.
If that 1000ft attenna was a high dB one the vertical beamwidth would be very narrow and it would not receive as well because most other signals would not be within it's beamwidth (like a boat right below it would not be in it's beamwidth but one out farther may). Any antenna on the same level would have good reception with it.

[wingtime]

OK I see it now..  I should have read up few more posts.    Sarcasm doesn't always work out the way you intend it when written vs spoken.    I guess thats why we have these little guys: