W5TOM All Band SGC Loop


 

Tuner and Bias "T" nailed to the tree. Service loop in ladder line visible


This antenna began its life as a single band 17 meter full wave loop matched with a length of 75 Ohm coax.   Since I had the all band vertical, I wanted an all band loop with its characteristic low noise and horizontal polarization.   This would give me the same diversity choice on all bands that I had on 17 meters. The tuner is nailed to the side of the tree at the 10 ft. level.   Since the loop was in the bifurcation of a large oak tree, the wind motion required the feed line to have a service loop.  This is why the 8 ft. of 2 inch home made ladder line was added.The top of the loop is about 40ft. and the bottom is at 18ft.

  The SGC tuners need 13v for operation, so I built bias Ts to send the DC and the RF down the same piece of 213 coax. The Ts in the shack are built into the wall outlets that I use for the coax connection to the outside world. The ones at the tuners are built into weather proof die cast boxes.   A two position coax switch allows fast selection.   There is as much as 2 or 3 S units between the two antennas at max.   The best choice for any given station and frequency could be either antenna, and sometimes there is no difference.   Generally the vertical is the noisier, but not all the time.   The vertical is usually best on 40 meters and below, but the real benefit is on 30 meters and up.   The advantage is on both transmit and receive, and the power limit is 200 Watts.


 

Here is the loop in the tree.  It is in the space between the two bifurcations of the tree. It is very stealthy.


 

 This is the schematic of the installation.


 

The 68 micro Henry inductor and the .04 micro Farad cap can be seen inside the waterproof box.


 

Here is the station bias "T" built on the wall outlets.


 

Technical Data on W5TOM   Bias T and Electrical Outlet Coax Connections.

  The bias T and the electrical outlet coax connections were laboratory tested for insertion and return losses at frequencies below 30 mHz.   A Hewlett Packard 8751 Network Analyzer with HP 87511 Perimeter Test Set was used by a qualified RF engineer and the results were plotted and printed. These traces follow this summary. The performance was better at the lowest frequency and slightly deteriorated as the frequency was raised to 30 mHz.   The average losses are quoted in this summery.

For the Bias T :    

Insertion Loss = 0.15 dB   or 0.025 S Units

Return Loss (SWR) = 1:1.03

For the Electrical Outlets:

Insertion Loss = 0.05 dB or 0.008 S Units

Return Loss (SWR) =   1:1.25

The combination of 2 ea. Bias Ts and 1 ea. Electrical outlet connection.

0.30 dB + .05 dB = .35 dB or 0.06 S Units

For the Electrical Outlets

This is predicated on the coax seeing 50 Ohms on each end and a practical power limit of 500 Watts. The power limit is suggested since the investigator was unable to evaluate the dielectrics used in such equipment.   Considerable experience over 30 years has been acquired with power levels as high as 800 Watts and no problems were observed.   It is also necessary to take measures to prevent the inadvertent connection of the plugs to the house current.

  For Reference:  

1,500 Watts RF = 275V @ 5.5 Amps  

1,000 Watts RF = 225V @ 4.5 Amps

100 Watts RF = 70V   @   1.4 Amps

 


 

 

You can see the service loop in the twin lead and the discharge device across the feed line.


 

 

This shows the wall outlets with the 13.8v line coming out under the outlet cover with fuse and connector.


 

The two position coax switch for quickly changing between the two SGC antennas.


So many Hams utilize poor non resonant antennas with mismatched 50 Ohm feed line and then use an amplifier and tuner (line transformer) to boost their signal.   A 1,000 Watt amp will give you a10 dB boost, that's 1.6 "S" units. I usually realize that by selecting the best antenna.  I sold my amp some time ago.  The QSO game is a two way street, the amp boosts the TX, but the RX is way behind the curve....can you say Alligator?  My system optimizes both directions and optimizes for polarity, radiation angle, lobe direction, and locality.   Since the tuner is at the feed point, the coax sees 50 Ohms on both ends and sends 100% of the power to the antenna.   With the tuner in the shack (I know that is where yours is, sorry!), the coax has 50 Ohms on one end and something really scary on the other end.   This mismatch turns a major part of your power into warm coax.  The last time I tried this, I discovered that DX stations aren't impressed with the temperature of your coax and therefore don't answer CQs.  


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