snipped-for-privacy@here.com hath wroth:
I presume snipped-for-privacy@here.com is the same as snipped-for-privacy@here.com so that this application is for a vessel in size anywhere between a dingy and an ocean liner. Therefore, I have no idea whether either antenna will be suitable for your purpose without a clue as to where it's going to be installed and roughly how it's going to be used.
Not so subtle hint:
- What are you trying to accomplish? What problem are you trying to solve?
- What do you have to work with? (hardware, software, versions, etc).
Anyway, I can always guess(tm). There's a very good chance that a 70" antenna will have worse performance. Comparing the two antennas, if the 70" were twice as long as the 40", it would only have 3dB more gain (minus some internal losses) which is good for 1.4 times the range at best. That's the good part and the ONLY good part.
The bad part comes in various sections.
- The bandwidth of such antennas goes down as the gain goes up. It's quite possible that it will work better at mid band (ch 6) and have somewhat higher VSWR at the band edges (ch 1 and ch 11).
- The vertical radiation angle will be less. The 70" antenna claims
- Both antennas have about a 3 degree downtilt. This is normally used on high mountain tops and tall buildings to avoid illuminating the sky. By moving the pattern downward, more ground area is covered. However, this is useless for a vessel where the antenna is mounted near the water level. It's also detrimental as nothing is free in antennas. Downtilt means slightly less gain (about 1-2 dB).
- Mounting is always critical with an omni. The problem is that there's no direction in which the antenna does NOT radiate. Therefore, any mount that is anywhere close to the antenna will have a huge effect on the antenna pattern. In general, you have to be at least the length of the antenna away from any metal masts, rails, rigging, bulkheads, etc. The longer antenna will need to be farther away. If you fail to observe such positioning, you'll have unpredictable holes and peaks in the horizontal pattern.
If you compare the vertical beamwidth to a smaller and simpler sector antenna that I recommended earlier in this thread:
Look at the 2nd from the bottom, "Inverted AMOS 9". You get 70 degrees of horizontal beamwidth so your vessel can swing somewhat at anchor. 16.4dBi of gain, which will more than either of your omni's. My guess is the vertical beamwidth is about the same at 5 degrees so if your unspecified type of vessel is sufficiently stable, it will be no worse than the omni with wave action. What this antenna does is trade horizontal beamwidth for gain. The other configuration offer different trades.
If you're going for the over the water DX record, you might want to look into the Fresnel Zone effects. See:
Basically, you need more than just line of sight for long distance at
2.4GHz. If we temporarily assume that the earth is flat, and that you're going for the 4 mile DX connection again, you will need to have both ends at least 37 ft off the water. Got that?If you must have an omni for aesthetic or religious reasons, I suggest it be a lower gain 8dBi omni, with an 11 degree vertical radiation angle, which will be somewhat less susceptible to wave action. If you're not sure, simply start some streaming video with a small buffer set. If the stream starts and stops in sync with the vessel rocking, then you have a vertical radiation pattern problem.