Type of aerial

[JimB]

I suggest that you go to the chandlers and see what they have got.
You are looking for a VHF marine band antenna.
Most of the ones I have seen are built into a fibreglass tube and have a coax connector on one end.
Mount it on your boat, connect the coax from radio to antenna and you should be good to go.

Try something like this:
https://www.electro-tech-online.com/custompdfs/2009/06/VHF-mast.pdf

JimB

 

[ccurtis]

For VHF on a boat, I would consider a J-pole antenna. It requires no ground plane as it is a half-wave antenna. They are easy to construct with copper water pipe. Search the internet to find all the construction details.

 

[unclejed613]

if i were to take a wild guess, the first one is a 27Mhz antenna (which if it belonged to a ham, could be easily cut for 10M (28Mhz)), the second one has more wire on it than i'm used to seeing on a 27Mhz whip, so my guess would be a 20M (14Mhz) whip, and the last one has more wire still, so i would guess it's a 7Mhz (40M) whip. remember this is a wild guess, only testing can tell what they really are. a dip meter or an antenna analyzer are probably about as simple a test as you will get. you really don't need a calibrated dial on a dip meter if you have access to a freq counter or a good general coverage receiver.

 

[unclejed613]

j-poles can be very cantankerous to tune but they work well without a ground, or you could make a 1/2 wave dipole with 2 elements about 13.2" per element. mounted in PVC pipe with a tee at the center, and you can mount the dipole vertically, but you must run the feedline perpendicular to the antenna for at least a few feet. the larger the diameter of the elements, the broader the SWR match will be, but using copper pipe has a limiting factor of weight. you will notice many VHF antennas are ground plane, colinear or discone antennas for this very reason. the ground plane acts very much like a dipole with a narrow tuning range, the colinear has a little bit broader tuning range, and the discone has a very wide tuning range, because the effective diameter of the radiating element is equal to the length (biconical antennas are even better, but more difficult to make). to have a large effective diameter, the element does not have to be a solid form. many large diameter antenna elements are made with "cage" construction. shown below are a cage antenna, a biconical antenna and a discone, just to give you an idea of the construction.

 

[Gregory]

What is the construction of the Vhf MME-101 or similar and how do they go about making this type of aerial.
Where do you begin to build and the process to achieve the working aerial

 

[unclejed613]

i can't find an MME-101 on the web.... who makes it?

 

[Gregory]

**broken link removed**
try this web address.

 

[unclejed613]

it seems their web server is on the fritz. i keep getting a 505 server error on their page.

 

[Gregory]

Here is a Print screen of the aerial if you have trouble seeing the print screen let me know and I will post the photo in sections.
Greg

 

[unclejed613]

ok, after a little poking around on their web page, i was finally able to get to the home page which (unlike the rest of their site) seems to be at least partially working. i was able to get a copy of their antenna catalog. the antenna you're asking about is a coaxial dipole.. a simple (well, sort of.. depends on the length, because as you'll see, making one is a bit problematic with frequencies lower than about 75Mhz) to make antenna. the simplest way to make one is to take a piece of coax, and strip the insulation off for about 10% longer than a quarter wavelength. then the shield braid is squeezed down the outside of the insulation, leaving an insulated center wire and a quarter wave or so piece of coax with the braid folded back over the insulation. the braid and the center wire are then trimmed to exactly 1/4λ (which is why it was cut 10% long, you lose braid length as it's folded back over the insulation because of it's increased diameter). the hard part is folding the braid back over the insulation because it has to be done a little at a time or the braid will get tangled with itself. a simpler way to make one is to get a piece of copper tubing a little bit larger inner diameter than the diameter of the coax, and cut the braid off leaving enough to solder to. strip off 1/4λ of insulation and braid leaving the insulated center conductor. slide the 1/4λ copper tubing over the coax and solder it to the braid. where the coax is stripped and the braid is joined to the braid, becomes the feedpoint, and the coax goes back down through the tubing back to the radio. the whole thing can be housed in PVC pipe or a fiberglass tube.

 

[Gregory]

Thank you I will try this out and let you know.
What size coax should I use any one will do ?
The frequency range is 156 to 163 MHz VHF
Greg

 

[unclejed613]

rg-58 is usually pretty easy to work with.... rg-8 is larger and will hold it's "shape" better in a PVC pipe. 75 ohm catv (rg-59) cable is cheaper and will work, but usually has a layer of foil stuck to the inner insulation which is a pain to strip off of the insulation, but the braid is easier to fold back, and the 75 ohm cable impedance presents a slight mismatch to a 50 ohm radio (although if the antenna is matched, the mismatched cable will still generally be less than 2:1 SWR, which is not great, but not bad (anything over 3:1 is usually not well liked by the radio)

if you need an "antenna on a spool", rg-174 is only 0.1" (2,5mm) diameter, and can be used with a plastic hook attached to the end for hanging the antenna and the antenna can be rolled up for storage on a small solder spool.