3. Antennas & Cables

3. Antennas & CablesEdit

First things first... antennaS for TV/radio applications, antennAE for arthropod appendages.

Antenna Types:

A variety of antenna options exist for VHF telemetry. To date, collaborators have used 9, 6, 5 and 3-element yagi directional antennas, and single-pole omni-, j-pole antennas. 9-element yagis have a long, very narrow detection range, whereas 3, 5, or 6-element Yagis have a relatively shorter and wider detection range.   Generally 9-element antenna are great for creating long, linear invisible fences to detect passing animals or to detect flights, or departures of animals from specific sites. Smaller 6, 5, or 3-element antenna are better for monitoring animals on the ground, or covering a broader airspace over a smaller area. Omni-directional antennas are best suited for determining species presence-absence patterns (e.g. seabirds at a colony), but not for providing directional information (e.g. departure directions of songbirds from a stopover site). Many projects may wish to utilize one or more antenna types depending on their research questions.

Estimated detection ranges based on biological data (simultaneous detections between two antenna from different stations):

9-element - Conservative estimate is in the 10-15km range. Maximum detection range in optimal conditions are probably 20-25 km - we've had numerous examples of simultaneous detections between towers more than 50km apart. However, when animals are on the ground, or roosting, detection range can drops to close to 0. Active animals in the front, or side-lobes of the antenna can be detected from a distance of 1-2km.

5-element - Conservative estimate is somewhere in the 3-5km for animals in flight. 1-2 km for animals on the ground.

Mono - Usually only

6 element antenna - somewhere between 5 & 9. 3-element antenna, somewhere between 5 and mono.

Coax Cables: 

Cable Types used to date include:

RG58 - run of the mill communications cable that typically come with a BNC connectors. Best used for lengths less than 50'. Cheapest.

RG213 - higher grade cable that can be used at length of up to 100' with low signal loss. Custom cable ends depending on distributor/manufaturer. Moderate price.

TWS/LMR-400 - similar to RG213, but higher quality (stronger weather/sun resistance) coating. Best for longer-term installations and long cable length. Most expensive. Manufacture can suggest which cable is better - LMR is usually cheaper. Has the lowest loss, especially over long distances.

Connectors. When ordering antennas and cables, it is important to ensure that 1) connectors between the coaxial cable and FUNcube, and between the antenna and coaxial cable, are compatible, and 2) the impedance rating of all cables and connectors is the same.  FUNcubes have a 50 ohm input impedance rating.  Therefore, to minimize signal loss all connectors and cables leading from the FUNcube to the antennas should be rated for 50 ohm.

Connecting to a FUNcube/RTL-SDR Dongle:

FUNcubes have a USB connection on one end and a female SMA connection on the other. One way or another, a cable needs to be stepped down to a male SMA connector to attach to the FUNcube. You can therefore use an adapter (e.g. BNC female to male SMA adapter), or you can have the cables custom made with a male SMA.  Note that Lotek units require a male BNC connection.

FUNcube with female SMA connection pictured next to a male BNC to male SMA adapter.

Connecting to an antenna:

9-element Yagis and omni-directional antennas tend to come with a female PL-259 connector (but this should be verified prior to purchase).  Options for connecting to a female PL-259 connection include:

1. Coax cable with male BNC connector at one end and male PL-259 connector at the other end.
2. Coax cable with a male BNC connector at both ends + a BNC female to PL-259 male adapter.
3. Coax cable with custom female PL-259 connector at the antenna end and a male SMA connector at the FUNcube end. (Option with fewest adapters and therefore less signal loss, but may be more expensive due to custom ends)

Ideally, the number of connections between the antenna and FUNcube is kept to a minimum so as to minimize signal loss.  From this standpoint, option 1 or 3 may be best.  However, there are also advantages to going with a more generic coaxial cable described in option 2, including being cheaper and more readily available (option 1 cables may need to be custom built/ordered). 

The 3 and 5-element Yagis usually come with a female BNC connector and so they can be connected to a FUNcube using a coaxial cable with a male BNC connector (with SMA adapter) at both ends.

Female BNC to male SMA adapter (cable to FUNcube) next to a male BNC to female PL-259 UHF (Antenna to cable).

*** For studies where comparing received signal strengths among antennas is important (to determine departure flight direction, for example), it is crucial that the types of connectors, number of connections, cable lengths, and cable types (different cables have different attenuations) be the same.  All of these factors can influence signal loss between antenna and FUNcube.  Therefore, if the antenna-FUNcube pathway is not the same for all antennas, observed differences in received signal strength among antennas may be due to equipment differences rather than the position of the bird/bat being tracked. ***

Mounting Antenna:

Antenna can and have been mounted to just about anything - see our Gallery here. However, just because you can affix them to anything, doesn't mean that the antenna will operate optimally. Most antenna are designed to be 'butt-mounted' extending outward from a structure instead of being affixed at their mid-point. However, this is not possible with many structures and can be logistically complicated and unsafe to install. Consideration should be taken to avoid contact or interference with other metal structures such as supports from a tower that the antenna is affixed to. Masts extending horizontally or vertically off a structure is a useful method.

Effort should be taken to mount the antenna as far apart as possible (~>1m), especially when they're oriented in the same plane, but this is not always possible.

There are several ways to mitigate this (Thanks to Casey Halverson):

1) increase vertical spacing as much as possible, greater than one wavelength is ideal.

2) you can place the antenna above or below 90 degrees in a different direction, this further increases the spacing between antennas that will actually interfere

3) butting up antennas to the mast, avoiding the stacking altogether (the preferred choice) -- they will be completely invisible to one another

Some additional insight from Bob Morton from Maple Leaf Communications has provided some more detailed notes on potential losses and the interactions between multiple antenna on one structure for antennas at different angles and antennas pointing 180 degrees apart.

Potential Suppliers:

• Maple Leaf Communications - info@mapleleafcom.com will supply 9-element, and/or J-pole (mono/omni) antenna and cables
• Economy 2-way distributing in the US - info@econ2way.com will supply 9-element antenna and cables together for about $500 each. -tends to have the best or comparable prices - used heavily by US partners and are familiar with the setup. Good customer service.
• Digikey.ca- www.digikey.ca - www.digikey.com
• Hutton Communication- 5228 Everest Dr. Mississauga, ON. L4W2R4 416-255-6063 - Canada - also has US distributors.