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Author Topic:   Multiple antennas on one HD Shakespeare SS mount
David Pendleton posted 12-02-2012 05:44 PM ET (US)   Profile for David Pendleton   Send Email to David Pendleton  
I currently have two heavy-duty stainless steel Shakespeare mounts on my hardtop. One is for my MORAD VHF antenna and the other has a short fiberglass mast with my Raymarine GPS mounted on it.

I would like to add a WiFi antenna (from IslandTime) and a dual-band 2m/70cm Ham antenna to my array (probably from MORAD if they can make it for me). I have room for more mounts, but would rather not go that way if I don't have to.

Does anyone know of a product that adapts the Shakespeare-style mounts to multiple antennas? Maybe you've built something yourself? Any ideas?

contender posted 12-02-2012 07:35 PM ET (US)     Profile for contender  Send Email to contender     
David, couple of things: I have never seen or heard one of these set ups you are talking about from Shakespheare Radio, However this can be easily achieve by mounting a flat plate to the 1st mount and drill a hole at two ends for the mounting of two antennas. Question: I believe I read once that antennas need to be apart by at least three feet to operate correctly (interference). Can you have a local welder weld on a flat plate to mount the antenna on some other place on the H-top? Or I think you are going to have to come up with a plan B....Good luck
Buckda posted 12-02-2012 08:22 PM ET (US)     Profile for Buckda  Send Email to Buckda     
David -

Since you have a mast for your Radome, you may choose to do something like this.

Mount the GPS to your mast, and the other for your WiFi antenna (or the other way around).

Just a thought. has the scanstrut light or gps bars available domestically. For about $175.

jimh posted 12-02-2012 09:17 PM ET (US)     Profile for jimh  Send Email to jimh     
Perhaps you can move the GPS antenna to the stern. It would be closer to the SONAR transducer, so there would be better correlation between water depth and position. This would clean up the hard top roof and make room for another antenna on that one mount.
jimh posted 12-02-2012 09:19 PM ET (US)     Profile for jimh  Send Email to jimh     
Dave--Browse around at this website:

This guy sells all kinds of antenna base, adaptors, mounts, and so on.

White Bear posted 12-04-2012 04:11 PM ET (US)     Profile for White Bear  Send Email to White Bear     
"Perhaps you can move the GPS antenna to the stern. It would be closer to the SONAR transducer, so there would be better correlation between water depth and position."

I could be wrong, but I am assuming we are talking about a BW, not the Queen Mary. Moving the antenna several feet should not make any measurable difference in one's daily operation.

jimh posted 12-04-2012 04:58 PM ET (US)     Profile for jimh  Send Email to jimh     
Space on the hard top roof of Dave's boat is scarce. That scarcity of space led me to suggest moving the GPS antenna elsewhere. I suggested moving it to the stern to be co-located with the SONAR transducer. This is a win-win situation:

--frees up space on the scarce roof top
--improves correlation between position and depth sensors

I don't see anything wrong with my suggestion. If you have a better idea for Dave to gain mounting space on the roof of the hard top, let's hear it.

contender posted 12-04-2012 05:11 PM ET (US)     Profile for contender  Send Email to contender     
Picture of daves top and mesurements would be better...
jimh posted 12-04-2012 05:18 PM ET (US)     Profile for jimh  Send Email to jimh     
Better than what?
David Pendleton posted 12-04-2012 08:40 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I don't have dimensions but here is a picture of my hardtop and it's installed antennas:

I've decided to scrap the idea of a permanently-mounted 2m/70cm antenna, so that leaves me with just finding a home for the WiFi antenna.

My GPS receiver can be surface-mounted, so I could probably move that outboard of either HD antenna mount, thus freeing up the port side mount for the WiFi antenna.

My other, and perhaps best, option is to use the light bracket sold by my RADAR tower manufacturer, Edson. However, my tower is a model from their "Classic" series and is no longer manufactured. They do still list the bracket as an accessory to their new "Vison" series of towers. I can't imagine it would be any different, but I have inquired via email about compatibility.

You can see the bracket here:

This option would cost me somewhere around $135.00 whereas my other option is free.

6992WHALER posted 12-04-2012 09:31 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
My GPS antenna is surface mounted on the hard top.
I have never had a problem receiving a signal because of the radar blocking part of the sky.
jimh posted 12-05-2012 11:37 AM ET (US)     Profile for jimh  Send Email to jimh     
In designing radio transmitter and receiver installations with a notion of being able to simultaneously receive and transmit without mutual interference there is a general rule that you need to keep the transmitters away from the receivers, both physically and in the frequency domain. A strong local transmitter can desensitize a nearby receiver or cause the receiver performance to decline due to intermodulation distortion products. A GPS receiver is listening for relatively weak--actually very weak--signals from transmitters that are at least 22,500-miles away at a frequency of 1.5-GHz. The RADAR unit is transmitting extremely strong pulses--perhaps with a peak power of several thousand watts--at microwave frequencies.

To give you a notion of the power level of the signals that a GPS receiver is trying to detect, modern GPS receivers are typically rated to have a sensitivity of -160 dBW. In terms of the more familiar rating of dBm, that is a signal of -130 dBm. Most boaters are familiar with the sensitivity of a VHF Marine Band radio receiver, which is usually stated in terms of microvolts. A signal of 1-microvolt will usually give reasonably good copy. That is a level of -107 dBm. From this we see that a GPS receiver is listening for signals that are as much as 23-dB weaker. That means signals that are 200-times weaker.

The transmitter of a RADAR is producing an output signal with a peak power of perhaps as much as 4-kilowatts. That is a signal level of +26-dBW. Compare that to the GPS receiver listening for signals as -160-dBW. The local signal is more than eight orders of magnitude stronger.

In light of the general principle of keeping receive antennas separated from transmit antennas, and considering the extremely great difference in signals levels, it seems to me that moving a GPS receiver antenna away from a RADAR antenna would be a good idea.

jimh posted 12-05-2012 12:04 PM ET (US)     Profile for jimh  Send Email to jimh     
I had to do some research to find the frequency band used by a typical recreational boat RADAR. It seems to generally be speicified as X-band, or 9.3 to 9.4-GHz. There is also a mention that some marine RADAR sets may operate in S-band or 2 to 4-GHZ. The X-band sets have quite a separation in frequency from the GPS main frequency near 1.5-GHz. However, all transmitters produce spurious radiation, and there is always the possibility of a RADAR transmitter creating spurious output closer to the GPS frequency. An S-band RADAR could be relatively close in frequency to the GPS receiver (L-band) signals.


In regard to interference, locating the GPS receiver antenna near the stern does put the receiver closer to the engine, and the engine could be emitting some noise from its spark ignition. Typically in marine engines there is a suppression system used in the spark circuit to minimize interference to radios. Without the suppression the spark ignition noise would affect many radio bands.

I don't think the SONAR signals will interfere with the GPS signals. They are very widely separated in frequency, and with the SONAR transmitter underwater, there is a great amount of attenuation of microwave signals through water.

David Pendleton posted 12-05-2012 12:13 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Moving the GPS receiver off of the hardtop is not an option as there is no more room in the rigging tube for the cable.
6992WHALER posted 12-05-2012 05:21 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
My understanding of GPS antenna mounting and radar is the best place is above the radar, if below the radar keep it close to the radar unit so the beam goes over it and dos not hit the GPS antenna directly

See page 4 of the link below pdf

jimh posted 12-06-2012 12:48 AM ET (US)     Profile for jimh  Send Email to jimh     
Dave--there is always room for one more cable in a rigging tunnel, but the cable has to really want to go through the tunnel.
dfmcintyre posted 12-06-2012 09:59 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
David -

I know this is really going to sound counterproductive, however...

As a test, put the antenna _in_ the hardtop box and measure the loss or dilution of position. You might be suprised.

Prior to mounting my Garmin's antenna, I would remove it from the console where it was laying on the floor and shock cord it to the console grab rail. Couldn't decide on mounting it on the already small console, or run the cabling up to the forward deck.

One day I noticed that the signal reception wasn't great, but readable and the DOP was 30 feet instead of the usual 10-15. Then I realized that the anteanna was _sitting_ on the floor of the console.

So I wedged it up underneath the console "top", just ahead of the flush mounted console. DOP is now about 20'.

That's where it's stayed for the last 11 years.

Regards - Don

jimh posted 12-07-2012 12:27 AM ET (US)     Profile for jimh  Send Email to jimh     
Mack--Dilution of precision is not really a product of GPS receiver location and signal strength. It is an expression of the accuracy of the fix deduced from the lines of position calculated to the satellites at their present positions. The more satellites you can see the better the chance that the position solution will be employing lines of position that yield a good DOP.

And on the effect of RADAR on objects that the transmit beam illuminates, I heard stories about the old 1950's DEW line RADAR sets being so powerful that birds learned not to fly in front of them.

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