Antenna Splitter for AM-FM Broadcast Band Receiver

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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Location: Michigan, Lower Peninsula
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Antenna Splitter for AM-FM Broadcast Band Receiver

Postby jimh » Fri Jun 07, 2019 11:02 am

A bashful correspond writes privately to me:

I am in the process of refitting my 1984 Montauk 17' and would like to install an AM/FM radio. However, I do not want to clutter up the boat with multiple antennas if avoidable. During my research I came across a Y-splitter whereby you can join an AM/FM broadcast band receiver and a VH Marine Band radio to one VHF antenna.

Is a Y-splitter a good idea, something feasible, or better left alone?

I do not want to diminish or harm the strength of the antenna nor harm the functionality of the VHF radio. Obviously the VHF takes precedence over an entertainment radio, and if needed I would install an AM/FM antenna. I appreciate you taking time to consider my question and look forward to your guidance on this matter.


I am posting this public reply because the topic has general interest, and my remarks may be of value to other readers.

All VHF Marine Band antenna splitter devices for AM or FM Broadcast band receivers are intended to permit an antenna that is resonant in the VHF Marine Band to be simultaneously connected to three devices:

  • a VHF Marine Band transmitter-receiver
  • an AM Broadcast Band receiver
  • an FM Broadcast Band receiver

The basis for this being possible is due to the difference in frequency among the three bands:

  • VHF Marine Band is 156 to 162 MHz
  • AM Broadcast Band is 0.5 to 1.6 MHz
  • FM Broadcast Band is 88 to 108 MHz

It must be understood at the outset that the antenna is only really resonant and thus proper for its intended band, the VHF Marine Band. The AM and FM devices are just receivers, and will not be transmitting into the antenna. The two broadcast receivers are sometimes combined to use a common antenna, although their significant difference in frequency and polarization makes use of a single antenna for both a compromise to start.

The function of the typical splitter is then to keep the 25-Watt transmitter signal from the VHF Marine Band radio out of the input to the AM and FM receivers, while not disturbing the connection between the antenna and the VHF Marine Band radio. These devices typically are three-port devices: antenna port, marine radio port, broadcast radio port.

There could be several ways to accomplish the division of the antenna port to other two ports, and how a particular splitter performs this function is difficult to know. The splitter devices are often not very expensive, which suggests to me that there must not be much circuitry or componentry involved in their design and manufacture. They also usually do not require any power for their operation, which suggests they use only passive circuitry.

If you look at the three frequency ranges involved, the VHF Marine Band is the highest range of frequencies. This suggests that one might be able to use a LOW-PASS filter to connect the broadcast radio port to the antenna. A low-pass filter does as its name suggests: it has a cut-off frequency and only allows signals lower in frequency to pass through without attenuation; signals higher in frequency will pass through but will be attenuated.

Assuming a splitter does manage to connect the broadcast port to the antenna port, and does so without affecting the marine radio port's path to the antenna, and has some method to attenuate the 25-Watt signal from the marine radio so it does not damage the broadcast receivers, we still have to consider what has been gained. Since the antenna is not resonant at either the AM or FM broadcast band, to know exactly how it will work for receiving those bands is difficult to predict. There is also the problem that not all VHF Marine Band antennas are constructed the same. There are many configurations for connecting the transmission line to the antenna in a VHF Marine Band antenna. Some antennas use capacitors or inductors in parallel or in series with the transmission line as part of an impedance matching network. The impedance of these components depends on frequency, and the impedance they present will be very different at 0.5-MHz (AM band) than at 156-MHz (Marine band). Without knowing exactly how the Marine band antenna has been constructed, how the splitter has been constructed, and the lengths and impedance of all the transmission lines interconnecting all the devices, it is impossible to know what sort of impedance will be presented to either the Marine radio or the entertainment radio. The resulting antenna that becomes connected to the broadcast band receivers is really quite a random antenna with unknown properties.

In reception of broadcast band signals, there is an anticipation that the signal strength available is going to be extremely strong. It won't take an antenna with much performance to pick up a signal. I think that is the real basis on which these splitters work. For local signals, a good receiver will have very good reception if the antenna is just a hunk of wire or even just a human body connected by touching a finger to the antenna terminal. You do not need much antenna in many locations.

My recommendation: before investing in a splitter try using just a short, dedicated broadcast band antenna, such as this $3.50 device:

https://www.defender.com/product.jsp?na ... id=4465443

This antenna may provide enough reception. If it doesn't, then you could investigate other options. Perhaps a small, flexible dedicated broadcast band antenna such as this $25 model:

https://www.defender.com/product3.jsp?n ... id=3745081

I don't like the introduction of any device in the transmission line between the VHF Marine Band radio and its antenna, on the basis that I consider the VHF Marine Band radio to be primary safety gear.

In regard to a splitter device called a "Y-Splitter", I would be very suspicious and very cautious in using such a device if, as the name suggests, the entire device is just a forming of three cables into a wye-connection. If a splitter were to be used, I would expect better performance to occur if the device, at the least, has some sort of enclosure in which there could be a few components. For an example, consider:

http://shakespeare-ce.com/marine/produc ... separator/

This device sell for about $60. I would assume that it can perform as advertised, on the basis that it has been on the market for a while, comes from a reputable manufacturer, and I haven't heard any groundswell of complaints about it. One vendor's website offered many positive comments. See

https://www.hodgesmarine.com/SHAKESPEAR ... 4357-s.htm

jimh
Posts: 6631
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Antenna Splitter for AM-FM Broadcast Band Receiver

Postby jimh » Sat Jun 08, 2019 12:45 pm

Regarding mentions that the inexpensive short wire AM/FM antennas have poor performance, there is one essential aspect of ANY antenna that will be used for reception of AM and FM broadcast: it should be vertically polarized if you want AM reception. If you get one of the three-foot wire antenna and just stretch out the wire horizontally, you will throwing away about 20-dB of signal, that means you will only get about 1/100th of the signal you could get if you oriented the wire vertically. I suspect that this accounts for many reports that these wire antennas don't work well.

For FM reception, horizontal polarization will work, but many FM stations also transmit a vertically polarized signal. If you want to get both AM and FM, orient the antenna vertically. The FM Broadcast band (88 to 108-MHz) was initially (c.1946) required to transmit horizontal polarization, and the FCC still requires higher-power commercial stations to maintain horizontal polarization, but for many years FM Broadcast stations have also been allowed to transmit and have been transmitting vertical polarization, too. The primary listening audience for FM broadcast are listeners in cars, and for many decades most car radio antennas were vertically polarized.