AIS antenna choice?

[tanglewood]

I've got an antenna choice to make for my AIS, and would like to get advice from the radio experts here.


It's a dedicated AIS antenna and I have an 8' extension pole to get it up as high as practical. On that extension pole will be the antenna. The antenna choices are:


1) A 4', 3db, "AIS" antenna that is presumably tuned to the center of the AIS channels.


2) An 8', 6db, "VHF" antenna that is presumably tuned to the center of the whole VHF marine channel range.


So it's a trade-off between gain, and more focused tuning.


Thoughts?

[scargo]

On my previous boat, I have installed an AIS transponder with a dedicated marine VHF antenna. And I did the installation in a such a way that I could use that antenna as a backup for my VHF radio in case something goes wrong with my main antenna and or cable.

[Dulls]

Good reply and good thinking.

[S/V Illusion]

Two comments -
1. There is no such thing as antenna gain. Antennas aren't active. The marketing takes advantage of the theoretical receive pattern which is only slightly altered by different length elements. Don't pay much attention to the marketing, and


Similar to the above, antenna resonance isn't particularly critical for receiving. Consequently, you will notice no difference between an antenna resonant on one specific AIS frequency and the frequency range in the same sector of the radio spectrum.



There is no trade-off

[tanglewood]

Originally Posted by S/V Illusion

Two comments -
1. There is no such thing as antenna gain. Antennas aren't active. The marketing takes advantage of the theoretical receive pattern which is only slightly altered by different length elements. Don't pay much attention to the marketing, and


Similar to the above, antenna resonance isn't particularly critical for receiving. Consequently, you will notice no difference between an antenna resonant on one specific AIS frequency and the frequency range in the same sector of the radio spectrum.



There is no trade-off

So you think both antennas will perform the same? If so, why would anyone ever used a longer antenna?

[S/V Illusion]

Originally Posted by tanglewood

So you think both antennas will perform the same? If so, why would anyone ever used a longer antenna?

There are essentially equivalent in any meaningful way. The difference between a quarter wave element and a half wave one alters radiation pattern, not gain and does practically nothing on receive.



The advertised difference of 3dB is theoretical isotropic "gain" indiscernible for all practical purposes and is actually a measure of radiation pattern which varies depending on heel angle so no "gain" can be quantified even if it did exist as boats move. Also, no difference in resonant frequency or impedance.


It's all marketing and quite effective as most people dont bother studying the intricacies of antenna design or performance.

[tanglewood]

Originally Posted by S/V Illusion

There are essentially equivalent in any meaningful way. The difference between a quarter wave element and a half wave one alters radiation pattern, not gain and does practically nothing on receive.



The advertised difference of 3dB is theoretical isotropic "gain" indiscernible for all practical purposes and is actually a measure of radiation pattern which varies depending on heel angle so no "gain" can be quantified even if it did exist as boats move. Also, no difference in resonant frequency or impedance.


It's all marketing and quite effective as most people dont bother studying the intricacies of antenna design or performance.

Thanks. Very helpful. I definitely represent the last comment, and just don't know enough about it, hence the question.

[Reefmagnet]

Originally Posted by scargo

On my previous boat, I have installed an AIS transponder with a dedicated marine VHF antenna. And I did the installation in a such a way that I could use that antenna as a backup for my VHF radio in case something goes wrong with my main antenna and or cable.

Same here. The VHF antenna on top of the mast, the lightweight racing whip AIS antenna on top of the stern mounted radar pole. AIS and VHF antennas can be interchanged if required in a minute or so.



I've posted before that the range difference with the AIS is about half the mast mounted antenna's range, but it's still good for around 30nm in open water.

[torrmundi]

Some misinformation here; antenna gain is indeed real and measurable. Ga (gain of antenna) is used in every link budget for comm links (terrestrial and satellite), every radar propagation calculation, every cellular base station coverage prediction, etc.

Ga may be relative to an isotropic point source (indicated as dBi) or, less commonly, relative to a half-wave dipole (dBh). These numbers differ by 3.1 dB, the gain of a half-wave dipole relative to a isotropic point source.

Yes, antenna gain is due to a shaping of the radiation pattern, by direction energy from unneeded directions into desired directions.

For sailing, omni-directional antennas are usually best as the direction of the boat usually changing. To accommodate heeling, the higher-gain omnis are avoided, as too much heeling will point the antenna's max gain at the water or the sky and the gain towards the horizon will be very low, compared to a lower-gain antenna with a broader vertical beamwidth.

3 dBi monopole antennas are common, as they are more compact than 6 dBi designs, but both will work. Gains of 9, 12, 15, 18 dBi are possible but usually reserved for land-based applications that don't heel.

Designs for VHF and AIS are simply a little broader in frequency of use; up to 162 MHz. Actually, the current published specifications for Shakespeare 5215 and 5215-AIS are identical, so I wouldn't be surprised of they only added a few words to the old specification: VSWR nominally 1.5:1 at 162 MHz.
Note that nominal doesn't mean much; maximum is better.

See some comparative frequency vs gain plots at https://www.cruisersforum.com/forums...-235427-3.html

Note the Shakespeare is measured at almost 1.9:1 at 162 MHz, not 1.5:1 as the marketing guy says.

[S/V Illusion]

Originally Posted by torrmundi

Yes, antenna gain is due to a shaping of the radiation pattern, by direction energy from unneeded directions into desired directions.

.

Bingo! It isnt "gain" but rather pattern distortion in a desired direction/take-off angle from a fized antenna. As we all know, boats move.


The term "gain"is really a misnomer and simplification by advertising types who recognize most people have no clue how antenna radiation pattern is varied so they over-simply.



All that really matters here is that people shouldn't buy something or pay more for something which implies greater efficacy based on misleading advertising.

[torrmundi]

It is not advertising. It is recognized engineering, present in every textbook on antenna design.

[smac999]

Originally Posted by S/V Illusion

Consequently, you will notice no difference between an antenna resonant on one specific AIS frequency and the frequency range in the same sector of the radio spectrum.

BS. an 8' shakespeare VHF antenna gives over 3:1 swr on AIS transmit freqs. some AIS will fault and stop transmitting. the 8' antennas have very small bandwidths around channel 16 and don't reach the AIS channels.

the 4' vhf anteannas have a wider bandwidth and are better with AIS.

on a sailboat you want less gain becuase the boat is always healing. so number 1 wins both for an dedicated AIS antenna.

[continuouswave]

Originally Posted by tanglewood

[AIS] antenna choices are:


1) A 4', 3db, "AIS" antenna that is presumably tuned to the center of the AIS channels.


2) An 8', 6db, "VHF" antenna that is presumably tuned to the center of the whole VHF marine channel range.

First a preface to my advice:

I have read many articles by "Tanglewood" and have observed he has a strong engineering background and likes devices to work properly. On that basis, I reply as follows, perhaps a bit lengthy but explaining the underlying priciples:

The primary concern for an antenna to be used for transmitting is to have good impedance match between the transmitter output impedance, the transmission line impedance, and the antenna input impedance. Measurement of the Voltage Standing Wave Ratio (VSWR) on the transmission line is an indicator of the degree of matching. A lower VSWR is a better match. Best is VSWR 1:1.

Many transmitters have internal monitoring devices to measure the VSWR (at the transmitter output into the transmission line), and often there is a protective circuit that will reduce transmitter power or even shut off the transmitter if the measured VSWR is too high. AIS transmitters typically have both a VSWR monitor and will shut down in the case of high VSWR.

On that basis, proper operation of an AIS transmitter requires a low VSWR at the transmitter.

Measurement of VSWR is best done at the point of connection of the transmission line to the antenna. The farther away from the antenna the VSWR is measured on the transmission line, the more loss in the transmission line will mask the real VSWR, that is, the VSWR will appear lower (better) than actual.

Many VHF Marine Band antennas are tuned for use in the very low end of the band; all ship transmissions occur in the very low end, generally never higher than about 157.4-MHz. The typical antenna is tuned for best VSWR at 156.8-MHz. The AIS transmitter operates at 162-MHz, at the extreme upper end of the band. (The AIS channels in use now are very closely spaced so there is no "center" tuning necessary.)

The VSWR bandwidth of an antenna is generally related to the antenna Q-factor, which most often is related to the diameter of the radiating element. Fat radiating elements have lower Q and produced less change in antenna input impedance with change in frequency, and thus have a wider VSWR bandwidth.

Now to the particulars of AIS transmitting antennas, if you are still reading.

If there is an significant transmission line loss between transmitter and antenna, the apparent VSWR bandwidth of the antenna improves when measured at the transmitter. It could occur that an antenna VSWR could be below 2:1 at the transmitter end of the transmission line even when the VSWR at the antenna was much higher.

In general, avoiding loss in the transmission line is highly desired, particularly when there is a regulatory limit on transmitter power. As a general rule, transmission line loss should be not more than 1 dB, or about ten-percent of the power.

Loss in transmission lines increases when there is a high VSWR, so to reduce transmission line loss, a low VSWR (at the antenna) is much preferred.

Since AIS transmission is already relatively low power, to produce the best transmitted signal, low transmission line loss is important.

If the transmission line length is not very long, that is less than 30-feet, and the transmission line loss is low, then an antenna tuned for the low end of the band will have VSWR greater than 2:1 at the AIS frequencies.

In summary, an AIS transmitter if being used with a dedicated antenna, should use an antenna tuned for the 162-MHz frequency of AIS.

All behavior of antennas is reciprocal on receive and transmit, so improved performance on transmit will improve performance on receive.

Regarding the radiation pattern of antennas and their gain. All antenna gain occurs by suppression of radiation in certain directions in favor of concentration of the radiation in a favored direction. In vertical monopole antennas radiation in the horizontal plane is generally uniform. Radiation in the vertical plane is not uniform, and this is where the pattern concentrations take place. To have any gain in one direction implies there must be a loss in another direction. For vertical antennas the usual goal is to have maximum radiation at the 0-degree takeoff angle, or at the horizon. To have gain requires the pattern to have nulls where the signal is decreased--often by a factor of 1/100.

The general thinking with ships is that larger and more stable ships can use higher gain antennas because their motion is a seaway will be less than smaller ships.

Antennas with advertised gains of "3 dB" have very little directivity in the vertical plane and are usually recommended for smaller boats. Antennas with advertised gains of "6 dB" are more directional and are usually recommended for larger boats.

All antenna gain is useful as long as the main lobe of the antenna is pointed at the desired remote station.

FINAL ANSWER: get an antenna for your AIS transmitter that is tuned for AIS.

Select gain according to your SHIP size, in this case, probably 6 dB.

Use low-loss transmission line, limiting line loss to not more than 1 dB.

ON FUTURE USES:
Two more channels have been allocated for AIS, designated ASM1 (Ch-2027 or 161.950-MHz) and ASM2 (Ch-2028 or 162.000-MHz). Using an AIS-resonant antenna will be useful for those channels. More at USCG NavCen

[continuouswave]

One follow-up thought on antenna pattern in the vertical plane for AIS:

Gain in vertical monopole antenna occurs generally by suppressing radiation at higher take off angles, that is skyward radiation, and usually involves a null at a 90-degree takeoff angle.

With global AIS ship tracking using satellite reception, if you wanted to increase the visibility of your ship on AIS by satellite, choosing an AIS antenna with lower gain would increase the radiation at higher takeoff angles, and probably improve the visibility of your AIS transmissions to overhead satellites. Conversely, suppressing skyward radiation as much as possible would likely tend to reduce AIS visibility to satellites.

[continuouswave]

Re behavior of antennas on receiving compared to transmitting: the antenna works the same way in both instances. That one can receive signals using a very poor antenna that would not permit transmitting is a valid observation, but it does not occur because the antenna works differently.

Transmission of signals usually involves a limited amount amplification of the signal when applied to the antenna. Reception of signals can employ almost unlimited amplification of a signal from an antenna. This accounts for the ability to receive signals on very bad antennas that could never transmit a decent signal.

The asymmetric of use of amplification in receiving and transmitted is seen in many radio services. Most transmitters are limited by regulation or in some cases by available power (as in satellites in orbit) to a certain transmitter power. Receivers can produce almost unlimited amplification to increase the strength of a received signal. Receiver amplification can overcome deficiencies in the receive antenna.