vhf radio

[davemaskell]

i've hooked up hundreds of radios but every once in a while i can't get reception unless i don't connect the shields together. i had a brand new raymarine 215 just today and my boss and i tried everything to get it to work . if you don't hook up on of the shields on the pl259 it work's great. everytime this happens my boss says let it go without the shields connected and in the four years we occasionally do this we never had one complaint. do you think it's in the antenna or in the radio. it works but i hate not knowing what's causing it.

[Alan Wheeler]

Dave, can you explain in better detail, what you mean by sheilds, with the emphasis on the fact you are suggesting two shields. What cable are you using?

[Rick]

Like Wheels mentioned there is only one "shield" to a Pl-259. Realize that the shield is merely the outer "half" of the coaxial transmission line and for proper transmitting output power it MUST be continuous (unbroken).

If reception improves with a break in the transmission line then that is symptomatic of not having an effective antenna at the end of the coax and you are then using the coax as a single wire antenna. Reception now happens but the efffective radiated output power will be terrible yet it may still be possible to communicate. I'm guessing that your customers with open "shields" are merely living with the problem and have not yet had to succeed at long distance VHF transmissions.

You probably are using antennas with base loading coils integral with the assembly and they require a metal "ground" attachment point like an aluminium mast. If the metal attachment point is not good you will get lousy reception (and merely some transmission).

The proper way to verify this it to use a Wattmeter (like a Bird Wattmeter with an appropriate VHF "slug") inserted between the radio and the transmission line. You should show near 25W (when the radio is set to high power) directional to the antenna with only a few Watts (or less) reflected power (back towards the radio). If you open the shield you will NOT get those readings. With a lousy antenna radiator installation you will NOT get those readings. With a lousy PL-259 installation you will NOT get those readings. This instrument is REQUIRED in order to verify a proper installation and NO competent marine radio should be considered acceptably installed without having made that measurement.

More info on that later if you desire..
Rick

[davemaskell]

that's what i thought. if you cut the pl259"s on the radio and on the antenna wire, solder the inner wire , tape it, and connect the shiields the radio won't work.
if you disconnect the outer shield it works.and yes the inner and outer are not touching. what would cause this. the radio or the antenna. there is no other break in the cable to the radio. or if you disconnect the pl259" and only have the inner male and female connected it works. connect the pl259 all the way and it won't work. my question once again is what would cause this.

[exposure]

PL-259's are difficult to install properly and all it takes is one strand of the shield to short out the center conductor and cause what you are describing. Disconnect the cable at both ends and check it with an ohm meter between the center and shield. It should read infinity. If it is proven ok, then you proably have a bad antenna. You do not want to leave the installation with the shield disconnected. It will reduce performance and may result in ruining the transmitter from a high SWR.

[Alan Wheeler]

OK, you are either confusing me more, OR if I understand what you are doing--YIKES!
First rule, NEVER, join Coax by the method of soldering and tapeing the inner and pulling the shield over and connecting that and tapeing. It destroy's the loading impedance of the coax. If you have to join coax, use a proper coax joiner. But best, never join it at all. Even with the use of a good quality joiner, you can lose 3dB of signal strength. 3dB is significant being half the power transmitted and 25% voltage loss.
Next problem. You may have an arial that is pre-loaded. Are you using a "Pacific Arials" antenna by any chance? These come with a set length of coax and you need to order the antenna with the length of coax you need. DO NOT cut or join these cables.
The best choice of PL259 is the one that has the screw into cable type connection. The PL can then be easily removed to run the cable and attached when finished. The use of these means no special crimp tool is required and the ones you have to solder are a pain and easily prone to shorts.
OK, the next point, NEVER, Transmit a VHF without the sheild or inner or no cable at all. You will destroy the output device.
Think about it, what is a sheild for? it is called a sheild for a reason. Otherwise we would use a two core conductor. Firstly, it is sheilding the VHF signal from disapating out through the cable itself. Remember you want all that transmitting power to get to the Antenna, where it will be strongly radiated in a correctly orientated polar pattern, due to the length and orientation of the wip.
Another critical point, is a "loading" is set up within the cable and hence you get a cable of a 75ohm rating for TV and 56ohm rating for VHF. Break that sheild and you have now destroyed that loading and the transmision point will now be at that break. Some of it may get to the antenna, but it's radiation strength will be poor. Plus the break in the cable is now radiating out VHF everywhere and possibly into other equipment as interference.
You maybe lucky enough to get reception and maybe transmission, but it will be far from what the system is capable of and chances are, you will eventually destroy the radio output.

[Rick]

Virtually all of the modern VHF marine radios have automatic SWR detection and shut down the output power accordingly. I've worked on many and have never been able to attribute the output power module failure to anything other than lightning hits or inadvertent application of external power supplies. This if fortunate for the radio owners that davemaskel has installed with open transmission lines.

In the range of 156 Mhz (marine VHF band) there is no magic about being able to add transmission line length to an existing antenna whether or not it comes with a length of coax. True, it takes some skill in being able to properly solder a PL259 yet this has been done successfully with good SWR results for over 50 years.

Davemaskell, I've already written above as to one reason why you have problems yet you need to have the equipment to troubleshoot the installation and verify proper results. That requires a Wattmeter/swr measurement device AND a 50 Ohm 25Watt dummy load. You can then make a measurement at the radio output AND then make another at the antenna. With the SWR meter at the radio output and the dummy load at the end of the coax you can verify that your radio's output, PL259 and transmission line are good. If they are good, then by placeing the SWR meter at the end of the transmission line with the antenna connected you can expect to get BAD SWR readings if you had to open the shield to get reception. Describe the antennas that you are connecting to and we can help further.

Rick

[davemaskell]

thanks for all the imput. i no longer work for the same company, that's why i wrote you because i new it wasn't right. do you have a web sight that i could check out a wattmeter. and what would you use for a dummy load. or could i just have a spare antenna to use for testing.