Another Full Day Of Radio!

SPE Expert 1K-FA
Cabling is nearly complete

Installation of the SO2R RX Loop Antenna TX Relay

The SPE Expert 1K-FA amplifier allows connection of two transceivers, four antennas, and, an SO2R RX-ONLY antenna. When transmitting on Radio#1, Radio #2 is connected to the SO2R RX antenna. This enables the operator to listen on Radio #2, on any band, while working the latest DX on Radio #1, and vice versa. Automatically. The Expert amp does all the switching.

SPE Expert 1K-FA Linear Amplifier

SPE Expert 1K-FA
Business end of the 1K-FA. Two inputs, Four outputs, Internal Antenna Tuner, SO2R, and it’s FULLY AUTOMATIC.

SO2R operation also presents the possibility of overloading or damaging the sensitive RX front ends of the two connected transceivers. The 1K-FA manual warns of this possibility and cautions the user about antenna separation etc.

W6LVP Loop Antenna

For an SO2R antenna it is suggested (and makes the most sense) to use a wideband RX antenna on this port. In my opinion the W6LVP Loop is the best antenna on the amateur market for this purpose. Perfect for use with my amplifier. I researched them all, and chose the LVP. The loop antenna is available in a version that includes a TX/RX Relay/Power Injector box. Wait, isn’t the SO2R port on the amplifier RX-ONLY? The answer is yes, but…and it’s a big but…only when the amp is turned on. Huh? When the 1K-FA amplifier is turned off, Radio #1 is connected to antenna #1 (which in this case is a resonant TX/RX antenna), and Radio #2 is connected to the SO2R antenna.

W6LVP RX Only Loop Antenna
The W6LVP Loop TX Relay has a dedicated PTT line from Radio #2

Found A Hitch!

Under the operating condition when the amp is turned off, Radio #2 will transmit into the RX-ONLY antenna. (!No Bueno!) And by doing so, would likely destroy the RX preamp in the loop antenna. Once I realized this I ordered the TX/RX Relay and put it place with a dedicated PTT line from Radio #2 to switch it. Even though the Relay Box can auto-sense RF, I felt a PTT line would better, perhaps even faster.  Some idiot-proofing by the operator, for the operator! I was sure it would only be a matter of time before I would make this mistake, so the relay was put into place.

Array Solutions - RXFEP
Array Solutions RXFEP – Receiver Front End Protector

During my research I found the Array Solutions RXFEP – Receiver Front End Protector. The RXFEP is a low cost solution and a must have ‘failsafe’ device to protect against overloading the RX in an SO2R configuration. After all, I’ll be transmitting with over 1KW output on an antenna in close proximity to the RX Loop antenna. The Flex 6000 series transceivers have RX front end protection built-in, the Flex 5000 does not, and one can never be too careful. Insertion loss? Not enough to bother me…

SPE Expert 1K-FA
The wiring is nearly complete. I know, I have to find a shorter 9 pin serial cable!

Next I will be adding the antenna connector breakout panels, an audio breakout, and the ground bar for the Single Point Station Ground.

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SO2R RX Loop Antenna

Todays work included installation of the SO2R RX Loop Antenna and associated devices. The W6LVP relay will provide switching for the SO2R RX Loop Antenna. I’ll also install some front end protection.

Here are some of the items on the agenda:

SO2R RX Loop Antenna
The wiring is nearly complete.


The SPE Expert 1K-FA amplifier allows connection of two transceivers, four antennas, and, an SO2R RX antenna. When the OP is transmitting on radio#1, radio #2 is connected to the SO2R RX antenna. This enables the operator to listen on radio #2 while working the latest DX on radio #1, and vice versa.

SO2R RX Loop Antenna for low noise

Operating in SO2R mode presents the possibility of overloading or damaging the sensitive RX front ends of the two connected transceivers. The 1K-FA manual warns of this possibility and cautions the user about antenna separation and it is suggested (and makes the most sense) that receiver front end protection devices be considered.

The Flex transceivers have built-in RX protection, but adding an RX front end protection circuit offers one more level of protection.

From the May 2014 QST article:

The RXFEP uses multiple devices to soft limit input signals starting at –1 dBm and outputting a specified maximum signal level of +10 dBm to the re-ceiver input.While manufacturers do not generally specify the maximum safe input levels for their equipment, it is worth noting that ARRL Lab re-ceiver dynamic range tests include input signals as high as +10 dBm, and we haven’t lost one yet.

An obvious place to install this device is at the RECEIVE ONLY ANTENNA port of a receiver. By having it as close to the receiver as possible, any pickup on the cable between the device and the receiver is minimized. There is another potential application in some installations, however. Some receive-only antennas include a remote preamplifier at the antenna termination. A protector at the remote preamp input would also be appropriate, although consideration should be given to the preamp input impedance, with a transformation to 50 W, if needed. If there is a long coax run from the remote preamp to the receiver, a second protector could be used at the receiver in case of loose coax connectors or other shield impairments. It should be obvious, but I’ll say it anyway. This unit cannot be used on any antenna feed that is also used for transmitting. Damage to both the unit and the transmitter would be likely consequences.

The low noise RX Loop Antenna is hereby considered a major station improvement!

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EA4TX AS2X2 Antenna Switch

One Step Beyond SO2R

For all intents and purposes, my SO2R operation is self-contained. I use a Flex 6700 dedicated to SO2R operation. I’m operating through an SPE Expert 1K-FA amplifier. This allows using two TX antennas and a wideband RX Magnetic Loop antenna. I enjoy fully automatic operation utilizing one Flex SCU (Spectral Capture Unit) per TX antenna. Really a perfect setup! This configuration has now been thoroughly tested and fine tuned to my operating preferences. Battle tested in contest with more multi band contacts than was ever possible for me while working manually. (the M word). In comes the EA4TX AS2x2 switch to enhance station operation.

I can now freely operate on any band at any frequency, and always have the lowest SWR and the appropriate drive level set automatically. We all know nobody has time to wait around for you to tune up during a contest! You’ve got to be there or be square!

Flex 6500: Dedicated To Digital

Now that I have the SO2R contest station squared away, I’ve turned my attention to the Flex 6500. I have MMSSTV running on 14230, EasyPal on 14233, and two instances of WSJT-X running on any two of any of the other bands that may be open, 24/7. I can work digital modes back and forth between any two bands with a click of the mouse.

For that matter I can also TX from any of the running programs with a single click thanks to the Flex DAX TX feature that switches the TX slice automatically. Combine that with a multiband antenna, and the possibilities are endless!

Watch That Front End!

So now that the digital station is setup and running what’s the problem? Well it’s not a problem yet, but very well could be.

The Flex radios have some built-in protection for the front end of the radio, but it would not be wise to count solely on that feature for complete RX front end protection. In the SO2R setup I added an Array Solutions RXFEP on the RX only antenna to protect the RX when the amplifier is not in operation to switch the RX only antenna automatically. When the amp is off, I have an extra level of protection.

With the 6500 running 24/7 digital modes it would not be hard for me to imagine accidentally TX ‘ing at 1,000+ watts while forgetting the other radio is nearby in RX mode (aka an ‘Inband Radio’). It would just be a matter of time before this type of accident would take place. I could easily make this mistake, so I must add a level of protection against it. In a multi-transmitter environment this is a requirement.

EA4TX AS2X2 Antenna Switch

EA4TX 2X2 Antenna Switch

I researched several antenna switches for the purpose of disconnecting the Flex 6500 antenna when the SO2R contest station is transmitting. Keep in mind I need to be able to transmit 100 watts through the switch. I quickly learned that the the first two “RX Antenna Switches” were not intended for use where the second radio is also a transmitter. Instead they were designed to switch out a receiver only. You cannot transmit back through the second RX port. So the first two switches were eliminated.

I came across the EA4TX AS2X2 and found it could handle 200 watts. It is not an RX only switch, it is a two port switch designed for switching (or exchanging) two antennas (TX and/or RX) between two radios. I’ll only need one side of this switch as I only need to disconnect the Comet CHA-250B antenna, not necessarily switch it with another.

I purchased the EA4TX ARS-USB rotor controller and have been very happy with its quality and performance, so another EA4TX product is welcome. Yes, I saw the single star review. I’m not worried because I know DX Engineering stands behind what they sell, as pointed out in the review.

UPDATE: The AS2x2 arrived today and is now installed at the antenna input of the Flex 6500. Whenever the Flex 6700 is in TX, the antenna to the 6500 is disconnected. Better to be safe than sorry.

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K6HR Contest Station Block Diagram

SO2R HF Contest Station Configuration

K6HR Block Diagram
K6HR HF Contest Station Block Diagram

Expert 1K-FA: At the Center of SO2R Ops

The Expert 1K-FA does all the antenna switching automatically. Fully automated, and carefully programmed, the 1K-FA greatly reduces the chance of user error. The K6HR Contest Station Block Diagram makes this SO2R setup a bit easier to understand.

The transceiver is a FlexRadio 6700 featuring two Spectral Capture Units. I utilize the two SCU’s as “two radios”, each with its own amplifier, tuner, and antenna. I designate ANT1 as the Mosley TA-33-JR-NW (for all HF bands 20 meters and above) and ANT2 as my 130ft Height Compromised HF Dipole for frequencies on 30 down thru 160 meters. Low SWR across all amateur bands!

The cool trick is the operator can move freely through the bands and operate at will via either radio. The 1K-FA’s foolproof switching ensures the TX radio will get the correct TX antenna every time. The second radio, in every case will be connected to the wideband RX loop antenna.

Data Driven via Flex USB

The Expert 1K-FA and the Palstar autotuners operate based on frequency data received from the radio. The 1K-FA and the AT-AUTO autotuners follow the radio in realtime as the operator changes frequency.

The operator only needs to turn the ‘tuning dial’ and press the PTT!

LP-500 Station Monitor

I have installed the LP-500 station monitor with 4 sensors. You could say it takes the guesswork out of getting good reports!

LP-500 Linearity Check

Leveraging the four sensors of the LP-500 Digital Station Monitor, I can ensure both amplifier channels are operating at full output, with the lowest SWR, and best linearity at all times.

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