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Old 05-18-2019, 09:47 AM   #1
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VHF Antenna Height

Two years ago I broke the tip on my VHF antenna. It had a solid wire tip with co-ax wrap up to nearly the tip. It was 8 feet. The break was about 18 inches down.
I taped it and it seemed to work but looked like a bent nail. It was atop an 11 foot base. I hesitate to call it an extension because it must have served a function other than raising the antenna. The bottom couple of feet of wire was encased in a small diameter aluminum tube kept in the very center of the base by a plastic washer with a center hole.Most of the rest was coax cable cushioned and positioned with a foam stuffing in the base. It terminated in a connector to the upper antenna which automatically joined as the two sections were screwed together.

Time to replace it.

With some difficulty I have removed the wire and padding from the base and would like to feed the new coax from the new 8 foot antenna into the base and simply use it as an extender for extra height.

I have two questions for this esteemed group. 1) Is antenna height the key to better, further reception and transmission? Since my antennas are mounted on my flybridge this combination will give me almost 30 feet from the tip to the water. 2) What was this old elaborate base all about?

Of course I could just skip reusing the old base at all and simply screw the new 8 foot antenna into the mount and run the coax to the radio but the "line of sight" theory seems to dictate the higher the better.

Comments please. pete
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Old 05-18-2019, 10:31 AM   #2
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I’d re-use the base. VHF transmits “line-of-sight”. Therefore, higher the better. The Coast Guard, for example, mounts their equipment on towers or mountain tops, if available. That’s why many times we can hear only the Coastie’s half of the conversation with another boat. The tips of my antennas are about 21 feet above the water. At times I wish they were 100 feet!
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Old 05-18-2019, 10:41 AM   #3
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When new to me, my boat also came with a tall "extender" as you have described. POs had traveled to areas with few other boaters, so longer distance communication was a focus. As the radios were older technology, as they were replaced, so too were the antennae. Now, instead of a forest of tall radio antennae, I have a single 8'(roughly) VHF antenna that mates with a single, dual station radio, a Standard Horizon with RAM. The extender is still in my basement, though I don't know why it hasn't moved to the landfill. My reception is excellent. The newer antenna and radio make a system that is excellent for good reception. If I wasn't boating mostly in the islands of the BC coast, but traveling on the open ocean where line of sight distances are greater, I might again want the extender, as simple math tells me I would increase my range (Google "height v distance to horizon", for the math). For you, whether or not to use an extender should also be dictated by the requirements of your location.
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Old 05-18-2019, 10:43 AM   #4
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Mostly true, all things equal higher is better but remember that various antennae have different wave lengths and a narrow wavelength is better on a lower-mounted antenna vs a wide wavelength which is better on a tall mast or a sailboat that will heel more. Make sure you get the correct wavelength for the relative height and stability of your boat. Go-fast boats with their antennae raked back always makes me laugh, just like our vaunted Prime Minister, looks over substance.
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Old 05-18-2019, 11:27 AM   #5
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Higher power overcomes weaker transmissions...so dB is important.


With enough power, even the curvature of the earth has not been an obstacle at times.


So it is a trade-off, height versus antenna dB. I like to have both at my disposal.


One or the other is just a roll of the dice in an emergency.
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Old 05-18-2019, 12:05 PM   #6
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Some of your range comes from radio quality (receiver sensitivity and/or transmitter power), some performance improvements come from antenna height and some from antenna quality. Then there's installation quality, i.e., the quality of the soldered connections, cable quality, proximity of the antenna cable to other wires and cables carrying electrons and other interfering systems like LED dimmers, other transmitters, etc.

Given all that, antenna height plays a significant role and, generally, the rule is 'the higher, the better.' It has to do with earth curvature and line of sight at long distances.
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Old 05-18-2019, 12:10 PM   #7
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The physics surrounding antennas generally confuses me.

You already have your new antenna so I’ll assume you have made a good selection that is appropriate for your radio and grounding system.

In general, the higher the antenna, the longer your line of sight will be. So sticking it way up high is a good thing. However, the longer the coax cable between your radio and the antenna to more power loss you will get. So a long wire run is a bad thing.

Not sure how much effect an 11’ extension would have but likely only about -1db at most. So if you are in open water such as the ocean or Great Lakes, then sticking it as high as you can may be a good idea. If you are in inland waters like I am, I’m not sure it would help you that much.
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Old 05-18-2019, 12:13 PM   #8
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Al...line of sight certainly is important for reasonable certainty of comms, but being a frequent user of low altitude VHF comms exceeding the curvature formulas....good radios with top notch antenna installs and high dB gain was my thankful result in great VHF comms when a long distance from land and hovering over vessels way out to sea....


My personal best was over 120NM from base in a hover over a cruise ship. Here on the east coast you routinely here USCG radios more than hundreds of miles away. Not a routine expectation, but with the right setup is it possible.
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Old 05-18-2019, 04:49 PM   #9
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Quote:
Originally Posted by dhays View Post
The physics surrounding antennas generally confuses me.

Well, Grasshopper, it's like this:


The reason we toss Gustav Hertz's name around every time we talk about radio is that he discovered that if you run an alternating current through a wire, a current will occur in a wire of the same length located elsewhere. In his case, the other side of the room. Hertz famously said, "It's of no use whatsoever." The point is, that ideally, the length of the two wires will match or very nearly so.


The wave transmitted is dictated by the length of the first wire, or as we call it, "the antenna." In our case, we are looking for a frequency (not wave length...wait for it) close to 158 megaHertz.



So to establish our wavelength, we plug numbers into C=lambda x f, or "speed-of-light equals wave times frequency. Needless to say, this produces some very unwieldy large numbers, but since they are mostly made up of zeroes, they become simple very quickly.


For our marine band, the wave is about 1.9 meters long...or tall, if you will. Most of us use a half-wave antenna, that is measuring from the mean to the peak...airplanes actually use a quarter-wave.


Now, the the answer to the original antenna height question can be derived algebraically, but it's simpler to use a cool tool called a nomograph--This example actually includes the formula in the fine print.


Pedantically yours, 'Prof
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Old 05-18-2019, 05:27 PM   #10
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Originally Posted by psneeld View Post
Al...line of sight certainly is important for reasonable certainty of comms, but being a frequent user of low altitude VHF comms exceeding the curvature formulas....good radios with top notch antenna installs and high dB gain was my thankful result in great VHF comms when a long distance from land and hovering over vessels way out to sea....


My personal best was over 120NM from base in a hover over a cruise ship. Here on the east coast you routinely here USCG radios more than hundreds of miles away. Not a routine expectation, but with the right setup is it possible.
US Gov't radios are top quality! That's great coverage.

Our HF radios were heads and shoulders above the VHF but then again, that's what they were designed for. They're also much more cumbersome to operate than VHF.
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Old 05-18-2019, 06:40 PM   #11
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Pedantically yours, 'Prof

I have been well schooled. Thank you!

Maybe you can answer a question that somewhat confuses me...?

Since the wavelength of the VHF signals that we use ranges is about 1.91xx meters, why aren’t full wage-length antennas generally used for Marine use? I am guessing that it is because using a dipole antenna gives more power (or sensitivity?) along the horizontal which is good for us?
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Old 05-18-2019, 07:12 PM   #12
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Alaska Prof only gets a B+, its actually Heinrich Hertz (1857–1894), a German physicist. An antenna must be a specific length to be resonant at the operating frequency, whether it's a transmitting or receiving antenna. It can be ˝ wavelength long with the AC voltage fed between one end of the antenna and ground. There is an image of the ˝ wave antenna reflected in the ground making it effectively one wavelength long, fed at the center. A TV “rabbit ears” is a 1 wavelength, center fed antenna. This latter configuration is not very practical for VHF use on a boat but is occasionally seen in specialized applications. The antenna can also be exactly twice as long and still be resonate. So a grounded, end fed antenna can also be one wavelength long. This makes the radiation pattern narrower in the vertical plane and thus has a higher “gain”, generally measured in decibels (dB). The transmitting and receiving antennas need to be parallel because of polarization. Vertical polarization is the VHF standard so the antenna must point straight up.

HF radios are not necessarily “head and shoulders above” VHF. For short range I would argue that VHF has less background noise, higher information carrying content and higher antenna gain than HF. Its limitation is that the range is limited to approximately line-of-sight.
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Old 05-18-2019, 07:49 PM   #13
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.... different wave lengths and a narrow wavelength is better on a lower-mounted antenna vs a wide wavelength which is better on a tall mast or a sailboat that will heel more. .
Vertical Beamwidth is the measurement of choice here.

Horizonal Beamwidth, for a boat, will be 360 deg.

Wavelength is not the measurement of choice, but does factor into the antenna element length; and some other stuff...

Yes, i am/was a radio engineer.
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Old 05-18-2019, 11:51 PM   #14
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Your antenna gives off electromagnet radiation similar to a microwave oven. Different wavelength and frequency but you can grasp the idea. You are in a dangerous place to be near the antenna when transmitting, or your crew is if they are enjoying a nice sunny day on the flyingbridge and you're below transmitting on the radio. The bottom of a 3dB gain antenna is supposed to be a minimum of 6 feet above the highest part of a person. So, 6 foot person plus 6 feet above that means the bottom of your 3dB antenna should be 12 feet above your flyingbridge.
Nope! I didn't make this up. Taken right out of the "danger/warnings" section at the front of a new Icom VHF manual. You know, the part no one reads!
For a 6 dB gain antenna, the most popular type on a pleasure boat I'm told, the height is much greater but I don't recall how much. Up to the skipper to make sure no one is in the danger
zone when tx.
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Old 05-18-2019, 11:56 PM   #15
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You can't really compare USCG land based radios with those on your boat. The land based units have a much higher radiated power, much better and higher antennas, and better receivers.

Be aware that the origin point of your radio "beam" is the effectively the mid point on the radiating element, not the highest point. Replacing a 4' antenna with an 8' antenna (using the same base) only raises the "beam" by 2 feet.
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Old 05-19-2019, 06:04 AM   #16
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Your antenna gives off electromagnet radiation similar to a microwave oven. Different wavelength and frequency but you can grasp the idea. You are in a dangerous place to be near the antenna when transmitting, or your crew is if they are enjoying a nice sunny day on the flyingbridge and you're below transmitting on the radio. The bottom of a 3dB gain antenna is supposed to be a minimum of 6 feet above the highest part of a person. So, 6 foot person plus 6 feet above that means the bottom of your 3dB antenna should be 12 feet above your flyingbridge.
Nope! I didn't make this up. Taken right out of the "danger/warnings" section at the front of a new Icom VHF manual. You know, the part no one reads!
For a 6 dB gain antenna, the most popular type on a pleasure boat I'm told, the height is much greater but I don't recall how much. Up to the skipper to make sure no one is in the danger
zone when tx.

This is from a Standard Horizon manual....


4 SAFETY / WARNING INFORMATION

This radio is restricted to occupational use, work related operations only where the radio operator must have the knowledge to control the exposure conditions of its passengers and bystanders by maintaining the minimum separation distance of 0.89 m (2.92 feet). Failure to observe these restrictions will result in exceeding the FCC RF exposure limits.

Antenna Installation:The antenna must be located at least 0.89 m (2.92 feet) away from passengers in order to comply with the FCC RF exposure requirements.


Wonder why the discrepancy? Articles on non-ionizing radiation seem to downplay the exposure hazards far more than the ICOM manual requirements.
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Old 05-19-2019, 06:26 AM   #17
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Since all marine VHF are limited to 25 watts of power, if you're planning a lot of remote cruising wouldn't it make sense to install a small power booster, perhaps bringing the power up to 50 watts or so? Legality aside, this could make a difference in a safety situation. I doubt today's modern antennas would fry with a bit more power through them.
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Old 05-19-2019, 08:54 AM   #18
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RG 213 Coaxial Cable

Once you've decided on the proper antenna don't cheap out on the coax. When I replaced my three foot whip antenna on my catamaran, a 65 foot or so run, I pulled out the cheap, pencil thin stock coax and replaced it with RG 213 coax cable. I received an immediate 10-12 mile increase in range on the ICW here in South Carolina. Stations such as Osprey Marina I used to have to be closer then 2 miles to be heard or hear from. Now, I can be heard at 10 plus miles away and hear them from 12 miles away. Huge difference.

Yes, it's big and fat and hard to pull, but at least you won't have to pull 65 feet. The price is not that much different from the small stuff. Just make sure you properly install the connectors, lots of info on the web for that.
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Old 05-19-2019, 10:01 AM   #19
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Since all marine VHF are limited to 25 watts of power, if you're planning a lot of remote cruising wouldn't it make sense to install a small power booster, perhaps bringing the power up to 50 watts or so? Legality aside, this could make a difference in a safety situation. I doubt today's modern antennas would fry with a bit more power through them.
Well, there are a number of emergency options. BUT first, DSC, being at 1200 baud, has a higher probability of reception, and as a bonus, the Lat/Long is part of the transmit data package. I suspect the advantage is greater than the 3dB ERP increase that any proposed 50W amp brick will give you.
Raising Tx power is a slippery slope, legally. I suppose you could buy a 120W VHF public safety radio with programmable power that is Part 80 compliant for marine use. But someone is going to have to know how to program this beast, with 1W, 25W and 120W channels with display nomenclature that makes it usable. OR, just keep the HP unit JUST for emergencies.

Personally, I carry VHF/DSC, PLB and HF/SSB on ham and marine. And soon to be, Airband. I have every chance to load up a very high $ VHF mobile, but, just don't see the need.
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Old 05-19-2019, 10:13 AM   #20
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Once you've decided on the proper antenna don't cheap out on the coax. When I replaced my three foot whip antenna on my catamaran, a 65 foot or so run, I pulled out the cheap, pencil thin stock coax and replaced it with RG 213 coax cable. I received an immediate 10-12 mile increase in range on the ICW here in South Carolina. Stations such as Osprey Marina I used to have to be closer then 2 miles to be heard or hear from. Now, I can be heard at 10 plus miles away and hear them from 12 miles away. Huge difference.

Yes, it's big and fat and hard to pull, but at least you won't have to pull 65 feet. The price is not that much different from the small stuff. Just make sure you properly install the connectors, lots of info on the web for that.
RG213 is a good choice. So is 9913 which gets you tinned braid. There are other choices in the size that contain foamed dielectric over plated Aluminum center, which I cannot recommend for a boat. I use a bunch of LMR400 at home, and it is lower loss than any of the above, but I would keep it miles from seawater. Too much aluminum and foam.
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