vhf not working properly

[fishcop]

My VHF does not seem to be transmitting well. Boats that are less than a mile from me say I'm faint and if they are a couple miles away they cant hear me at all. Is that a problem with the radio or the antennae?

[Glen E]

fishcop - 5/10/2006 10:49 PM

My VHF does not seem to be transmitting well. Boats that are less than a mile from me say I'm faint and if they are a couple miles away they cant hear me at all. Is that a problem with the radio or the antennae?

check all your connections first and spray them down with an electronic cleaner and regrease with dielectric grease....

[pbruce66]

Check everything that Glen says first, if you still have problems you can pick up an inexpensive thru-line VHF watt meter at West Marine, Boaters World, or Radio Shack and check your fwd and reflected power. If you have high reflected your problem is outside the radio (connector, cable, antenna), if fwd power is very low then your problem is in the radio.

[gonfish'n]

Yep all that has been said and take a close look at the connector making sure that NO shielding is touching the center conductor - if you are going to by a meter look for one with VSWR - You see if you have to much reflective RF you will decrease you transmission power - Let me explain -

The first concept to understand regarding antennas is that they are passive devices. To operate, they require no supply voltage. They do not alter nor process RF signals and they do not amplify RF energy. If they are 100% efficient, they radiate no more power than is delivered to their input terminal.

The basic properties that are used to describe the performance of an antenna include impedance and VSWR (Voltage Standing Wave Ratio). To better understand these properties and their impact on system performance.

Impedance and VSWR

In order to achieve maximum energy transfer between a wire or coaxial transmission line and an antenna, the input impedance of the antenna must identically match the characteristic impedance of the transmission line. If the two impedances do not match, a reflected voltage wave will be generated at the antenna terminal and travel back towards the transmission source. This reflection of energy results in a reduction in the overall system efficiency. This loss in efficiency will occur if the antenna is used to transmit or receive energy.

The resultant voltage wave on the transmission line is the combination of both the incident (source) and reflected waves (antenna). The ratio between the maximum voltage and the minimum voltage along the transmission line is defined as the Voltage Standing Wave Ratio or VSWR.

The VSWR, which can be derived from the level of reflected and incident waves, is also an indication of how closely or efficiently an antenna’s terminal input impedance is matched to the characteristic impedance of the transmission line. An increase in VSWR indicates an increase in the mismatch between the antenna and the transmission line.

Typically, most VHF wireless communications systems operate with a 50 Ohm impedance and therefore, the antenna must be designed with an impedance as close to 50 Ohms as possible. The antenna VSWR is then an indication of how close the antenna impedance is to 50 Ohms. A 1.0:1 VSWR would indicate an antenna impedance of exactly 50 Ohms. In many systems, the antenna is required to operate with a VSWR better than 1.5:1, why this is as frequency changes so does the impendence most antenna systems are designed to a mid frequency range low VSWR -

To indicate how increased VSWR impacts overall system performance the percentage of power reflected by the antenna, and the resultant overall transmission loss. For a 1.5:1 VSWR, the transmission loss is approximately 0.2 dB or a 4.0% reduction in efficiency. It is also important to note that some transmitter circuits decrease their output power with increasing antenna VSWR. This factor varies with each transmitter and is not quantified in this discussion.

Note: The term dB is a logarithmic expression of the ratio between two signal levels. For voltage the dB relationship is: dB = 20*log10(V2/V1). For power the dB relationship is dB = 10*log10(P2/P1).

When checking VSWR I always check it on the lowest and highest channel frequencies just to get and idea of the antenna efficiency -

Some other consideration is in the mounting of the antenna it should be as high as possible - perpendicular to the water and away from any metal - all of these consideration will reduce the efficiency of the transmission -

Hope this helps -

gonfish'n -

[Elusive]

I have one of the ART-2 testers. Call me if you want to use it

http://www.shakespeare-marine.com/el...s/switches.htm


Also, check & make sure the VHF didn't get switched to Low Power





Note to non-local THT members - we hear fishcop, we just don't want to talk to him

[Ludicrous]

gonfish'n - 5/11/2006 8:29 AM


Note: The term dB is a logarithmic expression of the ratio between two signal levels. For voltage the dB relationship is: dB = 20*log10(V2/V1). For power the dB relationship is dB = 10*log10(P2/P1).

I'm having problems with my Icom 502 and was going to ask some questions but now I'm afraid too.

[radioshop]

Dialectric grease is a bad idea. Use tape instead.

[gonfish'n]

Ludicrous Go ahead if I can answer it I will! - Don't let the dB = decibels formula scare you away! Just remember a little change in dB makes allot of difference -

gonfish'n -

[fishcop]

WOW again, awesome info here! Thanks for the replies. Gary, let me know nest time your on the shore. Maybe you can come over and we can check it with your meter.

[bbogue]

There may be a simpler solution. I had a similar problem a couple of years ago. Based on info from this forum, I found that my FF transducer cable was run too close to my VHF antenna wire. I moved them farther apart and also shortened the leftover VHF antenna cable and all is well. Good luck.

Bill