VHS Antenna Length

[bgbird]

What antenna length is optimal (or practical) for a VHS radio?

 

[cuzner]

Re: VHS Antenna Length

As long as possible, distance you can send is directly related to hight of antenea.<br /><br /> Jim

 

[Boatist]

Re: VHS Antenna Length

Sailboat 3to 5 foot 3 db gain. Power boats 10 to 30 foot 8 foot 6 db gain. 30 foot and larger power boats you may bennfit from antenna with a 9 db gain.<br /><br />Problem is as the gain goes up the horziontal radition angle becomes more narrow. So on a sailboat healed over you would be talking to fish on one side and airplanes on the other side but could not reach another boat on the water 10 miles away.<br /><br />Height makes some difference. Adding 10 feet may get you one more mile. Main thing is try and get the antenna it self high enough on the vessel that none of the antenna is blocked by super structure.<br /><br />Make sure you have heavy power wires to your radio. Use a good coax like RG8X. Use a good quality antenna. I think the Shakespeare Galaxy 5225 is one of the best. Made a big difference on my boat. Boat to Boat do not expect more than about 10 miles. Boat to Coast Guard with antenna on top of a 3000 foot high mountain you may get 80 miles or more.<br /><br /> http://www.westmarine.com/ecatalog/index2.html?rfx_ver=html <br /><br />I guess this link did not work the way I expected. Move to page 79 and enlarge for the west marine Antenna help page.<br /><br /><br /> http://www.westmarine.com/ecatalog/index2.html <br /><br />Go to page 79 where West marine has done some sutdy on antenna height and range and also antenna Gain and radiatation patterns. Enlarge page so can read.

 

[jurgenscraft]

Re: VHS Antenna Length

the length of a antenna used on vhf is related to the design of the antenna, is it a 1/4 wave ,1/2 wave or a 5/8 wave antenna, after correctly installing the antenna, as recomended by the manufacturer,the next most important thing to do, is setting the antennas SWR , thus enabling the maximum transfer of power from the transmitter to the antenna, VHF ,being mostly a line of sight communication frequency, it would be better to mount the antenna as high as physically possible, if you have a sailing boat, right on top of the mast would be the ideal possition, on a speed boat etc as high as possible on the boat or even a short extension mast will assist communications . always set your antennas for the lowest SWR.<br />Regards William Wright.

 

[captharv]

Re: VHS Antenna Length

Get the 5225 and be done withit. Best bang for the buck.<br />Captharv

 

[BLU LUNCH]

Re: VHS Antenna Length

To Boatist: Expain how do you get horizontal radition fron a veritacly mounted stick? You have problems from attennas being cross polarized horizontal vs veritcal is a -20db loss. Another problem is poor rf grounds. A poor rf ground causes major transmittting problems. Rg-8 is ok for up to 30mhz but not for 150mhz try 9913 with silver connectors YOU will see the differance .......Ham radio Extra Class from 1 land

 

[jlinder]

Re: VHS Antenna Length

Boatist was talking about the radiation pattern of the antenna as you look at different vertical angles.<br /><br />For everyone who does not understand antennas:<br /><br />Gain is the way you get a stronger signal by changing the way the signal is radiated. It is measured by comparing it to a standard, either a basic dipole antenna or an ideal radiator that has the same signal in all directions (called an isotropic radiator.)<br /><br />Typically you adjust the pattern so you do not send signal up or down, but in the horizontal directon. Normally this is a good idea, since you are not trying to reach anyone in the sky or down in the water below you.<br /><br />This becomes a problem when the antenna is tilted because your boat is listing, like on a sailboat in the wind.<br /><br />SWR stands for Standing Wave Ratio. On antennas, if they are not tuned to the radio and cable the power from the transmitter bounces back into the transmitter. Not only do you lose radiated power, but it puts a strain on the transmitter.

 

[jlinder]

Re: VHS Antenna Length

BTW, forgot to say that VHS is a video cassette. VHF is Very High Frequency.

 

[Boatist]

Re: VHS Antenna Length

Reply to BLU Lunch. Every vertical antenna has a horizontal radiation pattern (antennas 101). The radiator transmits at 90 degrees to the radiator. This is what we want in a boat antenna, (a omni directional horizontal pattern). What I was trying to explain is the horizontal pattern for different antennas. A normal 3 db gain antenna is 3 to 5 feet and has a horizonal radiation angle of about 80 degrees. This is a great antenna for sail boats as when they are healed over the radiation angle will still transmits to boats on both sides. Most Power boats can use a 6 DB Gain 8 foot antenna which has a horzional radiation pattern of about 35 degrees. With the 35 degree angle the signal has more range like a beam but still omni directional. The 8 and 9 DB antenna have a horizontal radiation of about 20 degrees and are great for large boats. but smaller boats running in ocean swells often pitch and roll more than 20 degrees and when talking to another distant vessel the signal will drop out. Also the 8 to 9 DB antenna can be 16 to 23 feet and not practical on small boats. I was not talking about horzional polarization but vertical Polarization.<br />VHF Marine Antennas do not require a ground plane and look to the transmiter as a 50 ohm 1/2 wave antenna. The antenna or feed line do not need to be tuned. I do alway check any system I install with a SWR meter when done to make sure have no shorted connectors. I have installed many and the very worst was 1.3 to 1.<br />Coax cable on a boat the RG-8X works very well. Even RG-8U is overkill. Most Quality VHF Marine Antennas come pre-wired with RG-8X and the cheaper antenna come prewired with RG-58. Yes on my ham gear I also use 9913 or 9914 but on the boats most antennas are no more than 9 feet away and the much smaller RG-8X works great.<br /><br />Ham licence you have me beat as I am only Advanced class from 6 land. I taught the extra class for two years at a local ham radio club but never wanted to learn 20 word per minute morse code.<br /><br /><br /> http://www.westmarine.com/ecatalog/index2.html <br /><br />Go to page 79 where West marine has done some sutdy on antenna height and range and also antenna Gain and radiatation patterns. Enlarge page so can read.

 

[Boatist]

Re: VHS Antenna Length

Jurgens<br />All the VHF Antennas I have seen appear to the transmitter as a 50 ohm 1/2 wave antenna. Some are actually 5/8 wave. They also do not require a ground plane. I have installed and troubleshot many VHF antennas. Every one I have seen required no antenna tuning or coax trimming. Worst SWR I have seen was 1.3 to 1. Most VHF antennas come with 20 or 25 feet of cable wired to the antenna. Just to find out I trimmed 6 inches at a time on one antenna coax to see how it affected SWR and it had no affect. Many boats do not need more than 3 feet of coax but I will not cut less 9 feet but have never had to adjust anything to get the SWR very low. Now I have seen many shorted connectors and a few cut/shorted coax cables with SWR almost full scale. <br /><br />Antenna height has also made very little difference. Boat to Boat with both antennas at 10 feet off the water (8 foot antenna mounted 2 feet off the water) the range is about 10 miles. Take one of the antennas and mount where top is 25 feet (15 feet higher) increase range to 11 miles. <br /><br />In the VHF band I have never seen a 1/4 wave antenna. Marine HF Single side band a much different story. Most seem to be 1/4 wave antennas and do require a good ground plane, antenna or coax tunning or of course a antenna tunner.<br /><br /> http://www.westmarine.com/ecatalog/index2.html <br /><br />Go to page 79 where West marine has done some sutdy on antenna height and range and also antenna Gain and radiatation patterns. Enlarge page so can read.

 

[jurgenscraft]

Re: VHS Antenna Length

TO BOATIST. thank you very much for the web site adress, it is amazing what a wide range of products that are available in the UNITED STATES OF AMERICA, what I was saying is summed up in the last paragraph of page 79.<br />kind regards William Wright

 

[phatmanmike]

Re: VHS Antenna Length

dont forget to add that just cause you have a 8 foot antenna, doesnt mean you have an 8 foot antenna. <br /><br />some people will make an 8 foot fiberglass rod, and only fill it 4 feet with wire, so you really only have a 4 foot antenna. fiberglass does not send and recieve anything....

 

[U_U]

Re: VHS Antenna Length

Thanks all. Most informative reading.

 

[Boatist]

Re: VHS Antenna Length

Jurgens Craft<br />I agree in antenna height should make a difference but it you look just above you will see raising antenna from 10 feet to 30 feet makes a difference of one mile. On my boat I tried a 4 foot antenna extension but on the water could tell no difference. Difference it did make was when fighting a double and working fish around the boat even with a 9 foot Salmon rod could not get over the top of the antenna. Off shore in swells lots more strain on the mounts. Traveling from the launch to the wash station much more likely to hit trees. When lay antennas down would extend beyond the back of the boat. Since I saw no difference I took the extension back off.<br /><br />If I left my boat at a marina and had a walk around deck I would try it again but with my little 21 foot Runabout Closed bow just causes too many problems and saw no inprovement. <br /><br />The is a big difference if 6DB antenna performance and Shakespears Galaxy 5225 is a great antenna. There are likely some better but it has become the standard for me.