VHF Antenna Fault ??

[kennyh]

Hi Guys

I think my VHF antenna maybe be badly wired (by the boat dealer), and I've already had to solder the connector at the VHF end.

I've checked the coax cable with a multimeter, and it appears there is leakage from the shielding to the coax core. Question: Is the sheilding normally connected to the coax core at the antenna (i.e. Should there be continuity between the core and the shielding when measured with a multi-meter) ??

Cheers
Kenny

 

[johnbo]

Re: VHF Antenna Fault ??

In most cases, there should be an open circuit between the shield and the center conductor.

However, it is possible that there is a matching network in the antenna base. If this was the case, I'd expect to see about 50 ohms.

Later,

johnbo

 

[Boatist]

Re: VHF Antenna Fault ??

Only way to check it is really with a SWR meter for VHF frequency. Yes it should come out 50 ohms impedence but you can not messure that with a regular OHM meter. If it shorted most of the time it is at the PL259 connector at the back of the radio. Many times people overheat the braid and it short out to the center core.

 

[Texasmark]

Re: VHF Antenna Fault ??

Agree on the overheating the cable when installing connectors.

As I recall RG-58 was 50 ohms impedance and RG-59 was 75 ohms. To properly operate the transmitter/receiver needed to "look out" and see a matching impedance cable and matching load. I know VHF Television used a 75 ohm balun so 75 ohm cable was required. Most radio's, as I recall were 50 ohm so they used the RG-58.

Note the word impedance, not resistance. This is a reactive circuit and you have a combination of inductance and capacitance, which at the frequencies involved make that value.

Agree that an SWR (Standing Wave Ratio) meter is how you tune one up....tune for max power/min SWR but that really tells you nothing about what power you are actually delivering.

For that you need a field strength meter and an antenna plot diagram and all that stuff...................or get a buddy some distance away and holler at him. d

Mark

 

[Boatist]

Re: VHF Antenna Fault ??

Top quality VHF antennas normaly have RG 8X, Cheaper one use RG 58 but both ar 50 ohms inpedence. Below is a link from Shakespeare that give the best answer to you question.

http://www.shakespeare-marine.com/faq2.asp?ReqNum=62

 

[jurgenscraft]

Re: VHF Antenna Fault ??

One of the best ways to test out a antenna system is to use a antenna analyzer, in the hands of a good opperator, or somene who has read the handbook a good understanding of the antenna system will be reached,
SWR meters only tell you half the story as they indicate the standing wave ratio at the point of insertion ( normally the SWR bridge/meter is inserted at the output of the transmitter via a short lead) the SWR at the antenna might be a totally different value. Try your local amateur radio club and try and have a helpfull ham come out with his antenna analyzer( SOMETHIMG LIKE A MFJ-259B HF/VHF SWR ANTENNA ANALYSER) and test your installation this particular instrumet will give you all the information you want to know about your antenna SWR, RESONANT FREQUENCY, IMPEDANCE.
VHF marine radio dealers / fitment shops should also be able to help you out . Now some of you may thinks that all of this is a overkill,well what is your boat, safety, and life andr the lives of your passangers worth, a good radio and antenna installation is what counts and maybe that little extra needed to make that "mayday" call get through to the coast gaurd.
Regards William Wright

 

[Boatist]

Re: VHF Antenna Fault ??

I been a Ham for many years and never used anything but a SWR meter to check the antenna. Marine antennas are not like many 1/4 wave antennas where you need the tune the coax length to get a low SWR. Any length of Coax over about 4 feet will give you a Low SWR if the coax is not shorted or the antenna broken. If you have a low SWR and the power meter part of the SWR meter show power out you should have no problem. Most SWR meters also have a Field strength meter built in also. Last call some one near your expected range, usually around 10 miles. If you can talk that distance it working good. Coast Guard or other land based stations with high mounted antenna the range can be more than 80 miles.

 

[jurgenscraft]

Re: VHF Antenna Fault ??

Hello Boatist
I like yourself have been licenced on amateur radioa for 30 years as well as holding a experimenters licence.but I like to do it right the first time, a four foot feed line with a dummy load at the end will also show a 1:1 SWR but will not radiate much rf.
Check the whole installation from output plug of rig to the tip of the antenna
Kind regards
William Wright