VHF question

[fairbank56]

Re: VHF question

Vhf is line of sight, It doesn't "skip".

There are basically two forms of long distance VHF-FM communications. One is sporadic E propagation which is a form of ionospheric skip. It is fairly common at different times of the year but much more common is tropospheric ducting primarlily caused by temperature inversions in the atmoshphere. It is almost a daily event in my area and usually occurs in the early morning up until around 9-10am. I routinely pick up stations well over the horizon up to 200-300 miles and occasionally much more. It's quite common to have check-ins on our local 2-meter ham (also VHF-FM) commuter nets from stations at these distances. In the ham world, sporadic E and Ducting are routinely reported and tracked.

Eric

 

[UncleWillie]

Re: VHF question

VHF is not line-of-sight.

VHF signals often experience skywave reflections or skip. It is very common.

But the phenomenon is random/sporadic and definitely not reliable.
Like a skipping stone, radio skip may work at 200 miles but not at the 30 mile range where you need it.
Murphy will guarantee that none of the range enhancing atmospherics will be present when the need is greatest.
Reliable communications are limited to less than 15 miles in most cases.

The gambler see a 10% chance of winning, the wise man sees a 90% chance of loosing.

 

[SteveMcD]

Re: VHF question

If you need to get scientific, yes VHF does skip under aberrant, brief, unreliable, and impossible for the average boater to predict conditions. A number of hams bounce off of the moon with helical phased arrays. I won't be getting one for my boat anytime soon. Since the vast majority of people here in this BOATING forum have absolutely no expertise or use for such things, perhaps we shouldn't confuse them with information they can't use.

 

[pdgs]

Re: VHF question

All frequencies can skip given the right atmospheric conditions. You can bounce SHF off the moon. The take off angle of the antenna is key in determining if it has any chance of skipping or not. Unless you bought a HF antenna your take off angle should be very low giving you an average clear communications range of about 20mi. The key is clear, if you are being blocked by mountains, garages, vehicles, other boats, etc. you may not get 1 mi.

Many antennas come with cable lengths that finalize the tuning of the antenna for an optimal SWR ratio when used in conjuction with a radio of the proper frequency range. These usually come clearly marked with instructions stating to not cut the cable. The cable is sheilded and DC (most common in boats) will not affect it. Flourescent lighting fixutures may affect it if it is layed on top of them. Excess cable can be loosely coiled and stowed. Tight bends will increase resistance and raise SWR which will adversely effect performance.

The most common problems found are a poorly assembled connector at the radio and a lack of bonding. The case of the radio needs to be cleanly bonded to the hull of the vessel as well as the antenna base. The connector needs to be made up with great care so that not one single strand of shielding is ever touching the center conductor.

A not so obvious problem is that during the process of attaching the antenna connector to the radio, the male end on the radio may twist if the female end is overtightened. This can break the small wire inside the radio and would require repair of the radio. Most marine radios are water resistant so repairs by the manufacturer are often the best choice.

 

[bruceb58]

Re: VHF question

I thought this thread had run its course last May.

Don't post to old threads.

 

[pdgs]

Re: VHF question

If you need to get scientific, yes VHF does skip under aberrant, brief, unreliable, and impossible for the average boater to predict conditions. A number of hams bounce off of the moon with helical phased arrays. I won't be getting one for my boat anytime soon. Since the vast majority of people here in this BOATING forum have absolutely no expertise or use for such things, perhaps we shouldn't confuse them with information they can't use.

Yup, did a lot of skipping with 500watts and a 35' FG whip. My personal best was from Subic to Wahiawa. I did one from Norfolk to Rota but lost it and never got the message out. Worst luck was going from Las Palmas, Canary Is. to Rota, danged Russians with their white noise generators docked all around us everytime we pulled in. That was fine with me, I'd just lock up the shack, hit the beach and go drink with them instead.

 

[pdgs]

Re: VHF question

I thought this thread had run its course last May.

Don't post to old threads.

Oxymoron.

 

[bruceb58]

Re: VHF question

Oxymoron.

Forum rules are not to post on threads older than 3 months unless you are the OP.