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ContinuousWave: Small Boat Electrical
Digital vs Analog VHF Radio Antennas
|Author||Topic: Digital vs Analog VHF Radio Antennas|
posted 03-20-2006 03:56 PM ET (US)
Inquiring mind needs to know (before purchasing)the difference(s) in VHF marine applications.
1. Is the line-of-sight transmission/reception improved.
posted 03-20-2006 05:52 PM ET (US)
You have fallen into the trap sprung by marketing nonsense. There is no such thing as a digital radio antenna.
posted 03-20-2006 07:34 PM ET (US)
Digital is a brand of antennas, that claims to have much better gain than the Shakespeare equivalent, in a 4' antenna.
The transmission technology is the same.
I have the Digital brand 4' antenna that is something like 4.5 DB gain, while the Shakespeare is only 3 DB.
I bought the Digital knowing 4.5 is better than 3.0 is with antennas. I hope it really is 4.5, either way it works well.
posted 03-20-2006 08:24 PM ET (US)
The original questions seems to imply there is some difference between antennas on the basis of digital versus analogue and seeks an explanation or evaluation. To repeat, there is no difference between antennas for the VHF Marine Radio Service on a basis of analogue techniques versus digital techniques.
Marine radio antenna manufacturers' figures for antenna GAIN are highly untrustworthy. There appears to be NO STANDARD for measurement. A recent test showed that many of these so-called gain antennas produce little gain and were outperformed by antennas which (correctly) advertised much lower gain figures.
The use of the word "digital" in the promotion and selling of VHF Marine Band antennas is an attempt to confuse people and to associate some favorable reputation of digital techniques with an antenna. There is no possible association of any benefit by use of digital techniques in the propagation of radio waves from an antenna.
Antennas sold under a brand name as "digital" can offer no benefit to improved performance. It is just silly marketing mumbo-jumbo, and anyone who has any grasp of radio understands this.
posted 03-20-2006 10:29 PM ET (US)
I was sceptical when the "digital" antenna Mfg. claimed a 9db gain with a 4' antenna vs Shakespeare's 6db claimed gain on their 8' antenna, hence my query.
Appreciate all the input.
posted 03-20-2006 10:59 PM ET (US)
For most gain comparisons, the standard used is the gain of a half-wave dipole antenna. The size of such an antenna varies with frequency. At the VHF Marine Band the frequency is 156-MHz and a half-wavelength antenna is approximately
Length of half-wave radiator
L = 468/F
468/156 = 3-feet long
In a so-called "4-foot" antenna, the radiating portion of the antenna is probably about a half-wavelength, and thus is cannot produce any gain (in comparison to a half-wavelength dipole).
There is often some confusion about vertical antennas of a half-wavelength in height having some gain, but this is in the case where they are operated above a ground plane. If operated above a ground plane of very high conductivity, it is possible for a half-wavelength vertical antenna to exhibit some gain (due to the creation of a reciprocal image antenna by the effect of the ground plane acting as a sort of mirror). Unfortunately, the gain occurs at a take-off angle which is not at the horizon. For this reason, the antenna length is usually increased to 5/8-th wavelength, which creates a better vertical pattern, and puts the gain back into a lobe on the horizon. But again, this pattern only occurs when the antenna is operated above a highly conductive ground plane. If the vertical antenna is elevated above a ground plane, or the ground plane is not very conductive, the image antenna effect diminishes.
On boats, these vertical antennas are not operated above a ground plane, and particularly so on fiberglass boats, and even more so when operating in fresh water, which, for all intents is not conductive at all.
In my opinion, there is more difference between antennas due to the design of the impedance matching network and the general quality of construction than there is due to factors of gain from longer length.
I just purchased a new antenna for my Whaler and will be installing it in a few weeks. I anticipate good performance from this antenna, based on its design, the wide use of it in military and commercial boats, and some test results I have seen which showed it superior. If all that obtains, I will pass on the details of the installation and antenna.
posted 03-21-2006 11:20 AM ET (US)
That would be great Jim, very enlightening post.
Out of curiosity, what antenna did you purchase? One of the Morads?
posted 03-21-2006 12:00 PM ET (US)
For VHF antennas, 0dB seems to be a handheld rubber ducky
rather than a 3' antenna.
And where was that "recent test" you referred to?
posted 03-21-2006 12:17 PM ET (US)
I have access to A.W.Rudge's antenna reference work.
Time permitting, I'll review it.
posted 03-21-2006 07:22 PM ET (US)
I started out with a Shakespeare 8' on my Montauk. It worked well but I soon found that it got in the way, I switched to a Shakespeare 3' ss whip and it was lousy. I went to a 4' Digital and it performed as good or better than the 8' Shakespeare.
posted 03-22-2006 08:16 AM ET (US)
I'm with Dick. Same boat, same radio, the same three antennae. Practical use showed 8' Shakespere fine; 3' Shakespere lousy; 4' Digital brand better than the other two. Have purchased, but not yet installed, a Digital for the 18. I've reported here before that the Digital produced some simply amazing results, picking up transmissions consistently from 20 miles away, and through areas blocked by stone bluffs. Don't ask me why the superior results, but they are real-world. The antennae were all mounted in the same place: the grab rail on the starboard side of the center console. Transmissions too seemed to be superior but I can't say for sure because no one ever listens to Harpoon Harry.
posted 03-22-2006 02:11 PM ET (US)
There are 8' Shakes and other 8' Shakes. What model?
posted 03-24-2006 08:31 AM ET (US)
You're right. And the 8' Shakespere I used was one of the basic models, not the $150+ model. The higher quality materials in the more expensive models probably make some difference. But having a superior performing, high quality 4' anntena is really nice on a small center console boat. Check out Digital's site (sorry, don't have the link.)
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