Antenna propagation pattern based on dbi

[dosw]

Currently I have three antennas to choose from for a masthead installation (I own these three):

 

MXTA-26: My favorite antenna. If this goes on my sailboat mast head I'll have to buy another of the same, though I don't mind. Advertised as a 6dbi antenna.

HYS SDN1T: Advertised as a 3dbi antenna. This one is surprisingly bad. More on that in a minute.

MXTA-25: Ghost antenna. For those who dislike this antenna, I'll say it works quite well. Better than the SDN-1T by a fair margin.

 

(Not in the running) Midland stock mobile 2.1dbi magnetic mount antenna. Because it's not NMO it isn't going to work on the masthead.

 

Sailing may be a problem for higher-gain antennas: While motoring or sitting idle, the MXTA-26 would be a fantastic choice for a masthead installation. However, under sail, I might be heeling 5 degrees to 25 degrees (often a long time at 10-15 degrees. So I'm trying to decide if this will be appropriate for the masthead. If it is not going to be appropriate, based on its radiation pattern, I'll probably have to go back to the drawing board and order something else.

 

Why not the HYS SDN-1T? I actually bought this one specifically to use on the masthead. But then I started testing it. Ugh, it's not good. Today I set up a voice recorder hooked up to a UV5G at home, with the MXTA-25 magnetically mounted to a window air conditioner at home. Then I drove four miles away, 350 feet below, with a couple of bluffs creating some line of sight issues, and tested several options.

 

1: The MXTA26 attached to my mobile Retevis RA87, testing at 5, 20, and 40w. In every case this antenna performed very, very well. At 5w the quieting wasn't as good as at 20w and 40w. But I couldn't hear any difference between 20w and 40w, and transmissions were crystal clear. Again, this is advertised as 6dbi.

2: The HYS SDN1T: Attached this to the same Retevis RA87, and again tested at 5, 20, and 40w. At 5w it was hard to make out. At 20w and 40w there was a lot of background hiss; not very good quieting. It just didn't sound very good. Again, this is advertised as 3dbi.

 

3: The Ghost antenna (MXTA25). I wouldn't mind putting this on the masthead. I'll have to repeat the tests mentioned here first. But it's in a plastic housing, and I don't expect that it would be as trouble-free sitting on a masthead for a few years. For now I'm ruling that one out just because I think its plastic housing would deteriorate over years of sitting up there exposed.

 

4: The "stock" Midland small magnetic mount antenna. This is listed as 2.1dbi. I connected this to another UV-5G and tested at full power (<5w). It sounded BETTER than the HYS SDN-1T, even compared to the HYS antenna being used at 40w.

 

So now we have a definite loser. The HYS SDN-1T is WORSE than the little almost throwaway magnetic mount antenna that Midland includes for free with all of its mobile radios. The conclusion: I can't put that on my mast.

 

But that brings me back to the original issue: Will the propagation pattern of the MXTA26 be awful when I'm sailing and heeling a few degrees? If so, I'll have to resume my search for a good masthead antenna that is more in the 2.1-3dbi range, that works with an NMO mount which I've installed on the masthead.

 

Is there an online calculator, or a formula that I can use, to see how propagation is impacted by antenna angle, or what the propagation is for a given angle above or below "level" for an antenna based on its gain?

 

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Update later in the day: I had some time to swap around and test using the HYS SDN-1T as a receiving antenna at home, and then comparing the MXTA25 to the MXTA26.

From one location about 4-miles out (where the MXTA26 previously had no problem getting through at any power from 5w to 40w), I compared the A25 and A26. The A26 performed very well, fully quieting at 15, 20, and 40w. Background noise at 5w. Then I swapped in the MXTA25. I was quite noisy at 40w, and from this location it didn't propagate well enough to break squelch at 5, 10, 15, and 20w -- only at 40w.

 

I tested again about 2 miles away and found the MXTA25 to be adequate at 5-40w with quite a lot of background noise at 5, 10, and 15w. And the MXTA26 was crystal clear at 10, 15, 20, and 40w, with slightly elevated background noise at 5w.

 

So if I were to rank their effectiveness:

MXTA26 - Very good simplex at 4 miles for 10-40w, some background noise at 5w.

HYS SDN-1T - Not great at 4 miles even at 40w. Usable but poor at 5w at this distance.

Midland Stock miniature magnetic mount antenna included with MicroMobile radios - About the same as the HYS SDN-1T symplex at 4 miles.

MXTA25 - Not usable simplex at 4 miles. Reasonable for simplex at 2 miles.

 

There's really no comparison, though. The MXTA26 outperformed the other three by a long shot. And I was surprised that the MXTA25 did as poorly as it did at 5w at 4 miles distance. The terrain isn't great; the receiving antenna is about 350 feet above the transmitting antennas, with several hills obstructing line of sight somewhat. Pretty good real-world test though.

[Guest]

id say your over thinking this.   get a good 36' or 48" no ground plane antenna  less money then replacing your 26 and they don't require a ground plane like the 26  or the ghost .   a 48" 6db gain antenna coupled with a good radio will do many many miles and put up with the salt air.   ^db wont be too narrow you will loose communication while under way but will still reach out.   a 9 or 12db antenna might be too narrow.  

[dosw]

Overthinking is my pass time. And it won't be done until the motor is done being serviced, which is a prerequisite to getting over to the mast crane to raise the mast. Until then I can experiment and play with options because it's interesting to me.

[SteveShannon]

This is an interesting technical question. An antenna over salt water which is just about the perfect ground plane. For one thing, propagation isn’t strictly determined as perpendicular to the radiator; the angle of the ground plane also affects it, albeit less so at UHF than HF.   
You might be able to simply raise a wire alongside your mast.

If I knew more about how to simulate this using one of the antenna simulation software programs I think it would help. Do you know someone who does that sort of thing? I just don’t know much about it. 
What type of antennas are typically used for marine band?

[tcp2525]

Stick with the MXTA-26 on the mast and stop worrying about the angle. It's not going to make a hill of beans difference. I know mine really performs amazingly well whether I'm standing still or doing 8 or 80. MPH in the truck. Only thing to worry about is to change the black set screws to a stainless steel one as salt water will corroded them quickly. 

[dosw]

1 hour ago, SteveShannon said:

This is an interesting technical question. An antenna over salt water which is just about the perfect ground plane. For one thing, propagation isn’t strictly determined as perpendicular to the radiator; the angle of the ground plane also affects it, albeit less so at UHF than HF.   
You might be able to simply raise a wire alongside your mast.

If I knew more about how to simulate this using one of the antenna simulation software programs I think it would help. Do you know someone who does that sort of thing? I just don’t know much about it. 
What type of antennas are typically used for marine band?

Marine VHF is in the 156-160MHz range. The common approach is, for power boats, a 6dbi or 9dbi antenna at the transom (smaller powerboats) or on the bridge (larger power boats), or a 3dbi whip at the mast head for sailboats. And the conventional wisdom is that a masthead 3dbi stainless steel whip will be more reliable under sail (while heeling) than a 6dbi or 9dbi fiberglass. I have a marine VHF, but it's mostly not legal to use it for ship to shore (not that anyone's going to care much around the Great Salt Lake, but this is an excuse to put up a GMRS antenna too) My rationale for putting GMRS on the boat is that (1) I can. (2) Repeaters are available. (3) Ship to shore is allowed on GMRS. (4) It's fun to try things out. I do have a 3dbi gain Marine VHF whip at the top of the mast, too.

 

My mast is about 29.5 feet long, and sits atop the deck which is about 4 feet above the water line. So 33.5' over the water from masthead to water line. My supposition is that is too great a distance between the antenna and the water to be able to consider the water a ground plane. But I could be wrong.

 

I do think I'll go with the MXT-A26 up there, and will use a putty-tape to seal around the set screws and around the connectors, along with some diaelectric grease anywhere water might intrude.

 

There are two repeaters that are pretty much ideal for allowing communications between the boat and my home. The Promontory Point repeater is 63 miles from my home, but situated very close to the lake, 2000 feet above it. I have line of sight from home to that repeater, and also from the boat to that repeater. There's also the Salt Lake Capital Repeater which is about 25 miles from my home, and 25 miles from the middle of the lake (in different directions). I also have line of sight to it from home, and the southern 1/3rd of the lake has line of sight to it. As you go North on the lake it's obscured by Antelope Island, though.

[tcp2525]

25 minutes ago, WRQW589 said:

Marine VHF is in the 156-160MHz range. The common approach is, for power boats, a 6dbi or 9dbi antenna at the transom (smaller powerboats) or on the bridge (larger power boats), or a 3dbi whip at the mast head for sailboats. And the conventional wisdom is that a masthead 3dbi stainless steel whip will be more reliable under sail (while heeling) than a 6dbi or 9dbi fiberglass. I have a marine VHF, but it's mostly not legal to use it for ship to shore (not that anyone's going to care much around the Great Salt Lake, but this is an excuse to put up a GMRS antenna too) My rationale for putting GMRS on the boat is that (1) I can. (2) Repeaters are available. (3) Ship to shore is allowed on GMRS. (4) It's fun to try things out. I do have a 3dbi gain Marine VHF whip at the top of the mast, too.

 

My mast is about 29.5 feet long, and sits atop the deck which is about 4 feet above the water line. So 33.5' over the water from masthead to water line. My supposition is that is too great a distance between the antenna and the water to be able to consider the water a ground plane. But I could be wrong.

 

I do think I'll go with the MXT-A26 up there, and will use a putty-tape to seal around the set screws and around the connectors, along with some diaelectric grease anywhere water might intrude.

 

There are two repeaters that are pretty much ideal for allowing communications between the boat and my home. The Promontory Point repeater is 63 miles from my home, but situated very close to the lake, 2000 feet above it. I have line of sight from home to that repeater, and also from the boat to that repeater. There's also the Salt Lake Capital Repeater which is about 25 miles from my home, and 25 miles from the middle of the lake (in different directions). I also have line of sight to it from home, and the southern 1/3rd of the lake has line of sight to it. As you go North on the lake it's obscured by Antelope Island, though.

At the distance to the top of the mast high quality feedline is a must. What are you using or going to use. This is more important than the type of antenna you use. A unity gain antenna will blow everything else out of the water if fed with good feedline compared to a high gain antenna with poor coax.

[dosw]

1 hour ago, tcp2525 said:

At the distance to the top of the mast high quality feedline is a must. What are you using or going to use. This is more important than the type of antenna you use. A unity gain antenna will blow everything else out of the water if fed with good feedline compared to a high gain antenna with poor coax.

I have LMR400 ready to fish through the mast, through the deck, and aft to the radio inside the cabin. I can do it in a single run with a water-tight through-deck fitting. My marine VHF uses RG8U, but at VHF frequencies that seems to be sufficient. My boat's GMRS radio is my 15w MXT275, recently put out to pasture when I got a Retevais RA87 to take its place in my vehicle.