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70CM BAND MASTHEAD PRE-AMPLIFIER KIT

Construction of a 70cm band Mini Kits EME221-70CM pre-a mp kit - November 2020


The Mini Kits EME221-70CM kit is a high performance masthead mount receiver pre-amplifier that is specifically designed for the 70cm band (430 to 450MHz). See below specification. I embarked on this project as I had had some good success working through the ISS repeater on a number of occasions, however felt that the receiver sensitivity of the Yaesu FT-736R 70cm module was not as good as would be ideal and a masthead mounted pre-amplifier would be a ideal solution. A number of fully constructed commercial pre-amps were considered, but finally the Mini Kits EME221-70CM pre-a mp kit was chosen based on the advertised performance specifications and dramatically more competitive price. 

The idea of the constructing a quality kit was also more appealing, if not a little daunting as I had limited experience with SMD (Surface Mount Device) construction; therefore best to get some experience.  

The kit notes do say 'Do not construct this Kit unless you are very experienced with surface mount construction'

Specifications

 

Frequency Range:

430 to 450MHz ( BW 20MHz @ 1dB )

Band Rejection:

Typically >35dB @ 145MHz, >60dB @ 900MHz

Gain Adjustable:

Typically +29dB Max, 20dB Min @ 435MHz

Noise Figure:

1dB ( Typically <0.9dB @ 435MHz )

Through Loss:

0.3dB @ 435MHz ( Bypass Mode )

RF Power Handling:

50W CW ( Un-Sequenced ) 100W PEP ( Sequenced )

Minimum RF Input:

+20dBm ( 100mW ) To activate the RF sensing circuit

Operating Voltage:

+11 to 15vdc ( +12vdc @ 220mA Recommended )

Board Size:

80mm L x 84mm W x 17mm H ( with TNC connectors )

Photo 1 Completed assembly of the PCB

The construction notes were excellent and the construction generally went smoothly except for dropping a small SMD resistor on the floor; it is still there, but will never been seen again. Once the replacement resistor was sourced the board construction was completed.

The pre-amp includes RF sensing circuity and relays that can operated up to 50W CW / FM and 100W pep SSB, however in this installation the DC power and switching will be injected into the coax cable via a Bias Tee and switched by the Yaesu FT-736R STBY Jack.

The tune up and performance testing was completed with a NanoVAN in LOGMAG mode that revealed the amplifier was producing 28.77dB gain at the target frequency of 436MHz. The NanoVNA required a 20dB attenuator between the CH0 port and the input to the amplifier to avoid overloading the amplifier.

Photo 2 Amplifier gain measurements with a NanoVNA.

The NanoVNA also showed that the 20MHz bandwidth (All the 70cm amateur band) was with 1dB of the peaked frequency of 437MHz. The gain adjustment range was also measured at +28.77dB maximum to approximately +19dB minimum, consistent with the specifications. The alignment of the amplifier is by tuning of the C2 trimmer for maximum gain at the desired frequency.

While on the bench a number of satellite beacons were monitored with and without the amplifier switched in. In one instant the satellite beacon was barely discernable in the noise and was bought up to S6 with the amplifier, not a totally scientific measurement as satellite signals are highly variable, however this did give a dramatic demonstration of what is to be expected. 

Fantastic kit, performs as specified, designed in Australia, look fantastic and enjoyable to construct.   

 

 

Video of the construction and testing of a masthead pre-amplifier kit designed for the 70cm band (430 to 450MHz). The video includes an overview of the kit, examples of my first attempts at the installation of surface mound devices and final performance testing.

 

 

Video is of a NanoVNA being used to measure the gain of a 70cm band pre-amplifier including the general set up and VNA calibration. This video is an exert from an earlier video detailing the full construction video of a Mini-Kits 70cm band pre-amplifier kit.

 

 

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Page last revised 31 December 2020  
 

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