Excellent explanation on antennas that I found most helpful and I thank the writer for his time and sharing his expertise.

Very practical and succinct summary of antennas. thankyou!

I was trying to find out the minimum distance between which I can mount 2 antennas, one for ais transponder and the other for vhf marine radio. I have one at the top of my mast and intend to mount the other about 2 metres underneath it on the spreaders of the mast. your article says to me that this should be ok due to the dohnut effect of the signal being horizontal. Please let me know if this isn't the case!

Boat-Project.Com Answer.
Neil; the "donuts" are pretty large, and the VHF free space propagation loss for 2m at 157Mhz is around -22db. This is simply for free space loss, and does not take into account the transmitting and receiver gain, or the radiation pattern of the antennas - which will change this figure. Thusly, you may see a large amount of signal at the AIS antenna from your VHF radio.

The short answer is that I cannot say for sure. This is one of those times that you may have to do a bit of "engineering" to determine for sure.

The main issue is whether or not the VHF transmitted signal will overpower the AIS receiver while it is transmitting. I have a concern that it could do so, simply because any weaker AIS signal the receiver is trying to pick up could be masked by the more powerful VHF transmitted signal (AIS and VHF Marine are essentially on the same frequency band). There is also a risk you could damage your AIS receiver due to it being overpowered by the transmitter.

The only way you would know for sure is to measure the signal strength of the VHF transmitter with a Field Strength Meter and see how strong the signal is at the AIS receiver's antenna - then contact the AIS hardware manufacturer to see what the receiver can tolerate.

If you do this, know that many Field Strength Meters are built for either Citizen's Band or Amateur Radio, and they might not give an accurate reading for the VHF Marine band. You'd have to find one that would operate at 157Mhz.

The second approach you might want to consider is there are several aftermarket switches that allow the AIS receiver and VHF Marine radio to share the same antenna. In receive mode, the two receivers simply share the antenna. But when the VHF radio transmits, the switch senses this and electronically disconnects the the AIS receiver from the antenna. It also grounds the AIS receiver's antenna lead so any energy from the VHF transmitter will not be sent to the AIS receiver and thereby damaging it. While the AIS will not be able to receive during the periods of transmission, unless you are transmitting a lot, the AIS receiver should be able to tolerate short interruptions of signal.

If your AIS hardware is both receiver and transmit, I am not sure if there are any switches of that type - as the ones I have seen are for AIS receivers only. You'll have to do some research.

I am probably erring on the side of caution, but you cannot know for sure without knowing the field strength of the signal at the AIS antenna location and what the manufacturer recommends. For liability-sake, I would caution you to be sure you are not damaging the AIS receiver.

Do you know the impedance for the coax 75 Ohms or 50 Ohms, or it depend on the radio?

Boat-Project.Com Answer.
Let me first start off by stating that coax has a "Characteristic Impedance", which is it's AC or working impedance. This is different than the cable's DC resistance, which would show an open (if you tried to measure from the center pin to the shield.

Typically radio systems are designed to work with certain impedance coax; in the case of VHF Marine radios, it would be 50 Ohms. Therefore you would want to use a 50 Ohm coax as well. RG-8, RG-8x, RG-58, and RG-213 are all 50 ohm coax, and are appropriate for VHF Marine radio use. In contrast, your typical home Cable-TV coax is 75 Ohm, and would not work very well on your VHF radio.

What about 52 Ohm coax? Well, some versions of RG-58 are 52 Ohms. Turns out that the core dialectric used in the Coax can affect the characteristic impedance somewhat, and low-loss versions of RG-58 may indeed be 52 Ohms. However, the difference between 50 and 52 Ohms is so slight that any mismatch in the radio/coax/antenna is not significant. If you recall from the discussion on this web page, there will always be some mismatch in the radio/antenna simply due to changing frequencies (since the antenna is cut to one frequency). Therefore, the slight mismatch introduced by 52 Ohm coax is not significant.

The antenna design subject is one of the most challenging designs for electronics engineers. The author did a very good job at explaining this difficult subject in an easy to understand format for the layperson. Thank you

Brian (Electronics Engineer)
Sep 15, 2011
Excellent clear explanations for beginners to experts well done