Antenna splitters and signal loss

[continuouswave]

Originally Posted by maclysaght

...one splitter separates out AM/FM, and the second splitter separates AIS and VHF.

Regarding the notion that a splitter might be able to create a separation among signals:

To separate AM Broadcast (0.5 to 1.5-Mhz) or FM Broadcast (88 to 108-MHz) from the VHF Marine Band (156 to 162) is quite feasible and can be done with rather simpler filtering because the three bands are in very different frequency ranges. The greater the separation in frequency, the more easily implemented the filtering becomes.

To separate two segments of the VHF Marine Band, the SHIP STATION TRANSMIT frequencies of 156 to 157.5-MHz and the AIS TRANSMITTER frequencies of 162-MHz is not really very feasible unless extremely sharply resonant filters are used. The physical size of these filters would be very much larger than the typical enclosure which houses devices marketed under the name of "splitters" or "combiners."

As I mentioned in an earlier reply, the commercial products provided which permit two transmitters to be connected to one antenna, such as a VHF Marine Band SHIP STATION TRANSMITTER and an VHF Marine Band AIS TRANSMITTER are really very fast acting automatic antenna relays that switch the antenna from being connected to both devices when neither device is transmitting, and then act to switch the antenna being connected to only one device whenever that device is transmitting.

I don't think that any sort extraordinary intellect is required to understand the basis of operation of the products, that is, no mind splitting is likely needed.

Regarding any notion of LOSS in these products, I already discussed the situation in which multiple receivers can be connected to a single antenna and with the use of an amplifier ahead of the splitting of the signal, the signal delivered to each receiver is no less strong than the original and may be even stronger than the original.

Devices that try to separate AM/FM from VHF Marine Band will likely introduce some loss because they generally must insert some sort of frequency selective tuned circuit into the signal path. The losses in such tuned circuits will depend on how many poles are used, the quality of the components, and the general design of the filters. Typically the losses are probably greatest in the non-VHF Marine Band paths, so the primary use signals are least affected.

Regarding any notion of LOSS during transmission, since the devices generally disconnect everything from the antenna but the single transmitter when that transmitter is transmitting, there is little loss in the device, other than the inherent loss of the transmitter signal having to pass through more connectors and relays. But the loss will be minimal, probably less than 1 dB.

[SobeFlyer]

I am grateful for very technical yet clear responses in these forums (i.e. continuouswave...).


I have a question along these lines... I want to install a dedicated FM/AM antenna atop my mast to maximize reception range of these broadcasts. To eliminate any potential losses, I am avoiding sharing antennas and installing splitters and prefer the one antenna per band being received. This makes sense to me to have an antenna sized for a specific frequency band intended, and direct wiring to a receiver/transceiver. I know this will require additional coax runs in the mast, but purity of design/signal is worth it in my mind, especially if trying to maximize reception range.


Questions:
1. Can an FM radio (87 - 106 Mhz) receiver be damaged by the transmission by a VHF-FM radio antenna installed close to it? (sharing the same masthead)


2. If so, would installing the antennas one above the other on the same axis (pole) reduce the possibility of damaging the FM receiver?


3. Would it make sense to install the receive antenna lower, say on a spreader?


4. Any risks to installing a Omni directional TV antenna near a VHF-FM transmitter on a mast?


I'm thinking every antenna should have it's own coax run. Thoughts?

[NPCampbell]

Originally Posted by maclysaght

Question: with two antenna splitters inline, am I losing signal strength? Am I better off re-attaching the dedicated Shakespeare rail-mounted antenna to the Icom AIS, and getting rid of the AM/FM/VHF splitter, and leading the mast antenna directly to my VHF?

It's been 30 years since my telecom classes but from what I can remember:

1) In a best case scenario, by definition, you are guaranteed to lose 3dB on each output leg for each unpowered 1-to-2 splitter.
2) The splitter needs to impedance match the two output legs to the input leg. The type of circuit it uses will cause additional loss. A resistive circuit will absorb additional power adding up to an additional 3dB loss per output leg. An improvement would be a narrowband circuit using a transformer which will only incur a .5-1 dB additional loss.
3) If you chain splitters, the losses are cumulative. If you split signal A into B+C and signal C into D+E then B and C will be a minimum of 3dB down each, and D and E will be a minimum of 6dB down each.
4) Unlike DC, there will be additional reflective losses caused by using cables/connectors of differing impedance and sharp bends or kinks in the cable.

[continuouswave]

Originally Posted by SobeFlyer

...I want to install a dedicated FM/AM antenna atop my mast ...


Can an FM radio (87 - 106 Mhz) receiver be damaged by the transmission by a VHF-FM radio antenna installed close to it? (sharing the same masthead)

The ability of a receiver to tolerate input signals at its antenna input that are extremely strong but not on the same frequency as the receiver is tuned to will vary with the particular receiver. In the case of a VHF Marine Band 25-Watt transmitter feeding an antenna, assuming zero losses and no antenna gain we would have a radiated signal level of

10(log)25/0.001= +44 dBm

The received signal level at another antenna assuming no gain would be a function of the separation distance. While perhaps not completely accurate, the free space path loss would be

dB loss = 36.6 + 20log(f) + 20log(d) (f in MHz, d in miles)

So at 156-MHz and 5.280-feet (or 0.001) miles the path loss wouldbe

dB Loss = 36 + 20log(156) + 20log(0.001)

Simplifying that would be

dB Loss = 36 + 43.8 - 60 = 19.8 dB

We would expect an input signal at the receiver to then be

+44 dBM - 19.8 dB = +24.2 dBm.

In terms of power in Watts, we subtract 30dB to convert to dBW, giving a signal level of -5.8 dBW, or about a quarter watt of power. Exactly how a receiver will react to an off-channel signal of that power is hard to predict.

Originally Posted by SobeFlyer

..would installing the antennas one above the other on the same axis (pole) reduce the possibility of damaging the FM receiver?

Assuming both are vertical polarized antennas, and assuming both antennas have a null in their radiation pattern at straight up or straight down, then the signal at the receiver would be reduced by the sum of those nulls. If each antenna has a -20 dB null, then the signal would be reduced by -40 dB. So we are now down to a -15.8 dBm input signal. Such a signal is a very strong signal but generally it would not cause any harm. A -15.8 dBm in term of Watts is -45.8 dBW or 0.0000263-Watts.

Originally Posted by SobeFlyer

Would it make sense to install the [FM Broadcast Band] receive antenna lower, say on a spreader?

On the basis that the higher antenna ought to be used for the VHF Marine Band, then, yes, the FM Broadcast Band antenna can be lower.

Originally Posted by SobeFlyer

Any risks to installing a Omni directional TV antenna near a VHF-FM transmitter on a mast?

This is hard to know. Usually TV signals are horizontal polarization, while VHF Marine Band is vertical polarization. The cross polarization gives another 20 dB of attenuation.

One way to test would be to connect the receiving antenna's transmission line to a small incandescent lamp, a miniature lightbuld like a No. 45 (3.2-Volt), then transmit on the VHF Marine Band radio. Carefully observe the lightbulb for signs of illumination. This method can be used with the FM antenna, too.

Originally Posted by SobeFlyer

I'm thinking every antenna should have it's own coax run.

If you use separate antennas then, of course, they will have separate transmission lines.

ASIDE: there are old stories about the AM radio station WLW transmitting at a carrier power of 500,000-Watts (with a peak-envelope power of 2,000,000-Watts at 100-percent modulation). It was said that farmers near the antenna site were stringing up antennas and rectifying the RF energy induced be WLW into the wires, then using that for electrical power on the farm. Apocryphal perhaps, but a good story.

ASIDE: I used to live only three miles for a NOAA weather radio broadcast station (KEC63) that was transmitting at 1,500-Watts into a 6 dB gain antenna, or about 4,000-Watts ERP. I had a VHF Marine Band antenna with about 6 dB gain that was pointed in the direction. I was able to receive very week signals on adjacent channels without any problem.

The path loss for KEC63 to my house would be

dB Loss = 36 + 20log(162.55) + 20log(3)
dB Loss = 36 + 44.2 + 9.5
dB Loss = 89.7 dB

KEC63 erp was 6,000-Watts +67.8 dBm

The signal at my antenna would then be
+67.8 dBm - 89.7 dB = -21.9 dBm

[NPCampbell]

Originally Posted by continuouswave

ASIDE: I used to live only three miles for a NOAA weather radio broadcast station (KEC63) that was transmitting at 1,500-Watts into a 6 dB gain antenna, or about 4,000-Watts ERP. I had a VHF Marine Band antenna with about 6 dB gain that was pointed in the direction. I was able to receive very week signals on adjacent channels without any problem.

My analog circuits instructor detailed a problem that he had when installing an amplifier into the larynx of his TAs to read vocal cord nerve inputs. They thought their amplifier design was faulty as the amount of noise picked up was incredible. However, one of the TAs noticed on a scope that the noise was amplitude modulated. This was later determined to be the broadcast from WBAP, a nearby 50,000 watt AM station. Adding filters and changing the lead lengths tuned the noise out of the circuit.

[continuouswave]

I was a bit sloppy in some calculations shown earlier. Better math follows:

In the case of a VHF Marine Band 25-Watt transmitter feeding an antenna, assuming zero losses and no antenna gain we would have a radiated signal level of

10(log)25/0.001= +44 dBm

The received signal level at another antenna assuming no gain would be a function of the separation distance. While perhaps not completely accurate, the free space path loss would be

dB loss = 36.6 + 20log(f) + 20log(d) (f in MHz, d in miles)

So at 156-MHz and 5.280-feet (or 0.001) miles the path loss wouldbe

dB Loss = 36.6 + 20log(156) + 20log(0.001)

dB Loss = 36.6 + 43.8 - 60 = 20.4 dB

We would expect an input signal at the receiver to then be

+44 dBM - 20.4 dB = +23.6 dBm.

In terms of power expressed in milliwatts:

P = 10^(23.6/10) = 229 milliWatts

--

For the path loss for KEC63 at a distance of three miles:

dB Loss = 36.6 + 20log(162.55) + 20log(3)

dB Loss = 36.6 + 44.2 + 9.5 = 90.3 dB

The KEC63 transmitted signal level in dBm would be

dBm = 10 log(6,000,000)
dBm = +67.8


The received signal would then be

+67.8 dBm - 90.3 db = -22.5 dBm

I should have noted that the receiver was able to copy very weak signals on adjacent channels, with signals levels around -100 dBm. This is a difference in signal level of 77.5 dB, which is very good adjacent channel selectivity and immunity from intermodulation distortion.

For some further comments about ability to receive weak signals in the presence of stronger signals see

http://continuouswave.com/ubb/Forum6/HTML/003356.html

[Feb2021]

Originally Posted by Brian.D

The best solution is to have dedicated antennas for whatever service you are using. Combining antennas for multiple services will invite unnecessary attenuation in the signal path. I direct your attention to police cars. Ever notice the antenna farm on the top of the squad car? Sure, they could combine antennas for their different service within the car, but that will just cause problems.

Best to keep AIS antenna to AIS system, VHF to VHF and AM/FM to that entertainment center.

JMHO

While this is generally correct and the resonance frequency of an antenna is always for one frequency (or more precise with negligible reflection in the near vicinity) and one frequency only resonant, there are smart alternatives like for example the discone antennas.
If you want to know more about antennas and antenna systems I strongly recoomend the standard
'Y3BK, Karl Rothammel Antennenbuch'.


Check it out.

But so far I have understood some of your gear is laying in water (bilge) and you have long cables running through the whole boat up to the mast? No good. While the connector have already losses (the higher the frequencies the higher the losses in general aka 'skin effect') the most important factor in reducing theem are the cables used. 'Aircom' produces neat cables with low losses in the upper regions.
But for lower frequency I'd always prefer symmetrical cables with symmetrical coupling instead of coaxial ones.


Cheers


Gee


PS: VHF is for pu***?