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Choosing the right WIFI or Cellular Antenna for your RV

December 12, 2015
Directional Antenna

Cellular and WIFI networks use a number of common frequencies that tend to be in the 800mhz to 5ghz range.  The most common and best known is the 2.4ghz range which most WIFI use.  WIFI has been changing in recent years to start using 5ghz frequencies as well.  Cellular frequencies tend to be more complex as every carrier tends to not use the same bands as each other, and they also use different bands for their 2G vs 3G vs 4G vs LTE and in some cases the same carrier may use different bands for LTE in different regions.  There are also different technologies and different ways to implement them leading to a large number of frequency ranges (Bands) being used.

Omni vs Directional

There are two different types of antennas that you could put on your RV:

Omni Directional, which are antennas that can receive signal from almost any direction.  These antenna usually radiate out in a donut shape perpendicular from the antenna.  This means when an antenna is vertically mounted there is limited signal propagation above or below the antenna but strong signal propagation horizontally in all directions.  Also Omni antenna tend to be limited to about a theoretical 8db gain max, while most stock antenna on most Wireless router are about 3db gain.  It is difficult to engineer WIFI antenna that can exceed 8db gain despite some manufacturers claim.  Omni antennas are far easier to mount as you can put it anywhere and it is always pointing the right direction.

Directional antennas, are antennas that offer much higher gain (thus longer range) than Omni antenna.  There are different styles that offer different gain levels and different aiming windows.  Panel Antenna tend to have lower gain for directional antenna but are much easier to aim with as much as 180 degree window to aim them.  Yagi antenna on the other hand have a very small window to aim but very high gain.  Since these antennas require aiming you would need to know where to aim, which can be fairly easy for campground WIFI but very tricky for cellular.  I have seen some demonstrations of the difficulty of aiming long range WIFI, in one case 2 Yagi antenna had to be aimed from roof tops with Rifle Scopes, as they were 5 miles apart.  Directional antennas can have gain of up to 25db providing they are aimed accurately.  They also can have as little as a 15 degree window (vertically and horizontally) to aim.

Different Antenna are tuned for different bands and may cover one or some bands you need but not all.  They may also perform differently on each band.  For example the Wilson Cellular Trucker antenna does give a few additional db of gain over their little stubby antenna that comes with many of their amps.  However the gain is different for each band/carrier with some bands coming in at up to 6db gain while others loosing up to -2db.  Keep in mind that you need to read the literature carefully as some antenna may advertise an 8db gain, but is that 8 additional gain above the gain of the antenna that ships.  In the case of WIFI, most routers ship with 3db antenna on them, so when buying a 8db omni antenna, you do not get 8 additional db but rather you add 8db after subtracting 3db from removing the existing antenna, for a net gain of 5db.

Cable Grades

LMR-100A

About 1/8″ diameter, ideal for under 3′ length, loss per 10′ with 2.4ghz WIFI about 4db

LMR-195/RG-58

About 1/4″ diameter, ideal for under 15′ length, loss per 10′ with 2.4ghz WIFI about 1.9db

LMR-240

About 3/8″ diameter, ideal for under 25′ length, loss per 10′ with 2.4ghz WIFI about 1.3db

LMR-400

About 1/2″ diameter, ideal for under 200′, loss per 10′ with 2.4ghz WIFI about 0.8db

*There are a lot of other grades but they are less common.  I have however seen recently a number of cell amp manufacturers starting to use common cable TV RG59 or RG6 cable with standard cable TV connectors.

*Exact loss can also differ depending on the brand of cable.  Generally brand name cable manufacturers like Belden or Hubbell will perform closer to these numbers and cheaper brands that you may find at a big box home improvement store may perform more poorly.  Cable loss also varies depending on the band you are trying to use, but generally for any cellular band the above numbers will work as a good approximation as the 2.4ghz for WIFI falls near the middle of a fairly narrow range of bands used by Cellular.

There are a number of common connectors used for Cellular and WIFI such as N, SMA, and TNC. There is even some unusual connectors like RP-SMA (RP= Reverse Polarity) used for early generation routers as a feeble attempt by the FCC to prevent home users from adding custom antennas to their routers.   However for real world application, LMR100 or LMR195 will fit most types of connectors needed for most home antenna, routers and amps.  LMR240 and 400 tend to be used for commercial grade applications not home use and are much more difficult to manage due to their size.  Adding additional connectors/splitters/extenders/pigtails to antenna cables will also cause additional loss, typically about 1db per connector.  Ideally for most RV applications, a custom  made LMR195 cable cut to the exact length needed is the most practical option.  It gives a reasonable amount of distance, will accept almost all connectors without the need to add pigtails or adapters and is still small enough to get thru tight voids in the trailer and keeps the loss to a minimum.

Cable length

When designing an antenna setup for your RV, cable length is the most critical choice you can make (the shorter the better).  Cable length is far more important than height or location.  Most people may think that having an antenna up a very tall mast would be better.  Taller is often not better due to the additional loss from the cable will significantly offset the gain from raising the antenna higher.  In the case of an 8db Omni antenna, using just 10 feet of LMR100 cable from the antenna to the amp/router will cost you 4db (half your gain).  Using 20 feet will cost you all the gain, making the antenna virtually pointless and that 20 feet will likely not be enough to clear a building or hill anyway.  And using more than 20 feet will actually cost you, as you will be loosing more from the cable than the antenna is gaining you.

Antenna placement

Antenna Placement is another important factor in a good RV antenna design.  Most RV’s, due to the materials used for construction often act like a Faraday cage (big cone of silence for WIFI or cellular).  After antenna cable length, having a clear line of sight is the most important factor.  Window mount antennas sometimes can help, but some glass that is glazed with tints and mixed with metals can also have an effect on signal.  Ideally getting the antenna outside would work best as it will have clear line of sight in all directions.

The best design is to carefully find the shortest and most direct route from the amp/router to the antenna thru your RV even if it means making tough choices like drilling holes, with the goal of getting the antenna outside the skin of the RV and not much further.  Every foot will cost you a good amount of gain.  The most innovative placement I have seen is installing the antenna to the Batwing TV antenna, so when it is cranked up it also get it another 3 feet of height above the roof of the RV and when the Batwing is down so is the Omni.  In my case I was able to do this solution with only 6′ of LMR195.  After my install and testing I was getting about 5db of additional gain which was exactly what I factored into my calculations with an 8db antenna minus about -3db due to cable loss.

WP_20150516_15_32_46_Pro

8dbi Fiberglass Outdoor Omni Directional antenna

WP_20151202_08_55_23_Pro

25dbi Yagi Directional antenna

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From → RV Tech

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