VHF Antenna Splitter Breakdown?

[Brian D]

The way my mind's eye sees this is very simple. The only way to effectively protect your FM Broadcast (FMBC) from RF energy while using your VHF is to isolate it from the path. This can be done with the below circuit. It is very simple and I will explain.

As shown, the VHF is keyed which energizes the relay in the path of the FMBC. This removes the harmful voltage and current from the path to the FMBC. The filter after the relay is a bandpass filter which will block out all VHF transmissions by about 60db. This will protect the front end (receiver circuits) in the FMBC.

When the VHF is in standby mode, the relay is released and connection to the FMBC is established, allowing both radios to share the antenna. One thing is missing is an impedance matching device between the relay and the filter. This is used to match the 50Ω antenna to the 93Ω or 600Ω input of the FMBC. It would depend on the type of FMBC you are using.

This is all theory and as a solution to the problem, I think it is possible.

 

[Chief RA]

Try hooking your FM receiver to your shroud lines. Recievers are passive and actually like long wire antenna for signal pickup. Its worth a try. Chief (FCC lic#1890)

 

[shemandr]

I don't understand anything about radio. Passive, active … you might as well be speaking Greek. My question is how would you hook up your FM receiver to a shroud? Solder, tape, twist?
And, can anyone tell me why I can't get AM reception (Hence Yankee games) on my boat while I can get it in my car 15 yards away?

 

[Chief RA]

It could be that AM is not received because your antenna system is that poor. Just wrap the center lead of your antenna coax around a shroud line and see if it works. Passive in this case means you are not transmitting RF power, just receiving. Let us know how it works. Chief

 

[Brian D]

I don't understand anything about radio. Passive, active … you might as well be speaking Greek. My question is how would you hook up your FM receiver to a shroud? Solder, tape, twist?
And, can anyone tell me why I can't get AM reception (Hence Yankee games) on my boat while I can get it in my car 15 yards away?

One highly probable reason is your AM/FM radio does not have an AM antenna in it. It probably relies on an external antenna, like the one in your car.

 

[Chris Patterson]

Thing is, you can strip back the outer core to expose about 19 to 21 inches of the inner core, and you may not be able to tell the difference between whether you have an antenna on it or not. Just lay it down behind the radio..

 

[Chief RA]

Right Chris, thats why I suggested multi wavelength antenna (shroud line)testing since it is recieve only. Chief

 

[Forrest15112]

I don't understand anything about radio. Passive, active … you might as well be speaking Greek. My question is how would you hook up your FM receiver to a shroud? Solder, tape, twist?
And, can anyone tell me why I can't get AM reception (Hence Yankee games) on my boat while I can get it in my car 15 yards away?

Andrew,
Typically when active or passive is used in anything electronic, it refers to whether or not it has a powered circuit. In this case, "active" should refer to a powered amplifier circuit, which is common in devices such as splitters, to recover the signal loss created by the splitter device. Some are extremely well designed and highly effective, and most consumer grade are so-so at best.

To radiate or receive efficiently, an antenna has to be of a certain length with respect to the frequency used. Basically, radio waves travel at the speed of light ( 300,000,000 meters per second). A wavelength is the distance ( at speed of light) it takes the signal to make one complete cycle. The formula is wavelength ( in meters)=300,000,000 / frequency. Here's the trouble with Am & FM using same antenna. Wavelength of broadcast FM, say 100MHZ = 3 meters, AM broadcast, using 1000KHz = 300 meters.

1/4 wavelength antennas are common resonant antennas and is what Chris Patterson explained by stripping back the braid on your coax. I'd bet Chris is a ham because 19-21 inches is resonant in the 2 meter ham band. Ideally, you would cut the coax back for the band you're wanting to receive. That works out to be 29 inches at 100MHz, but at AM broadcast,1/4 wavelength is closer to 250 feet! Even better would be to keep the braid attached, and stretch it out in the opposite direction forming a dipole. This would push the 1000KHz AM antenna to almost 500'. Stereo manufacturers compensate for this with circuits or wound wire antennas to create a resonant antenna on AM. There's a trade-off in performance, and this is probably why you can't receive AM on your boat. Think about antenna position too. Is your receiver below or near the waterline? Since most of the AM antenna may be in the radio, that's a poor location. AM is very susceptible to analog noise from electric motors, transformers, inverters, etc, and that could be worse in a busy marina too. Assuming you have a typical car stereo type receiver, it's designed to use a standard FM broadcast antenna and if you get that straightened out, you should get AM broadcast through it too. You could still have a noise issue or damaged receiver from a marine VHF transceiver/splitter.

Connecting to the shrouds? yeah, i 'd say solder, tape, twist, etc - whatever works. The shrouds are probably not resonant either and are probably grounded. There's a reason SSB backstay antennas are isolated from the rest of the rigging. If you're receiver is designed for a 100MHz antenna ( 3m wavelength), adding more length will not necessarily help reception.

 

[Chief RA]

Well Forrest, I understood it but probably not many others. Chief