TX & RX on a single site

Mariner2011

Hello, dear Flex Community. We have a requirement to install MF-HF receive and transmit stations at a single site. As I understand, 500 watts of output power from the transmitter (Barret 4050 with 30 meters multi-wire broadband dipole antenna) may interfere with or even damage the receiver (Flex 6700 with ATU horizontal dipole antenna). Please kindly suggest the bandpass or other filters, which may protect the receiving equipment and decrease the ambient noise. Also, if anyone used RX and TX with 500 watts of power at a single location - if any tips and notes on that? Thanks! Leo

KD0RC

Hi Leo, preselectors like these tend to be a bit fiddly. If mis-adjusted, your signals of interest (maritime emergency traffic) could be completely lost, and no one would even know it. I would not recommend them; in fact, for your application, I would recommend against them.

The Flex 6700 has a very robust receiver designed to be used in a high ambient RF environment, so there is a good chance that the worksheet will show that you can operate as intended.

One other thing to keep in mind - the Flex 6700s can be used remotely over the internet. The operating position can have computers running SmartSDR using SmartLink so that the operator could listen to the 6700s just as if they were physically in the same room. I mention this so that you have an alternative to contemplate if the worksheet shows that you cannot get your antennas separated enough.

KD0RC

Hi Leo, Generally speaking, running another transmitter in the vicinity of the 6700 will not cause any issues. That being said, it is possible to get the antennas very close together which could cause damage. Flex has a worksheet that you can use to see if you are in the safe zone. The worksheet is intended for Full Duplex operation of a Flex radio, but can be used for a separate transmitter as well.

https://www.flexradio.com/documentation/flex-6000-fdx-power-calculation-worksheet/

As to your last question, I have used 500 watts at Filed Day where we had two Flex 6400s with horizontal doublets within 100 yards or so of each other. The antennas were more or less in the same plane, and broadside to one another. We were not in any danger of damaging either radio, and in fact, could operate without mutual interference until we got quite close in frequency to one another. If we were on different bands (even harmonically related bands) there was absolutely no interference.

The big thing for your installation will be antenna proximity and orientation. The worksheet will walk you through all that. Please don't hesitate to ask any questions here if you have trouble with the worksheet.

Mariner2011

Thanks for your detailed answer! I will go through the provided worksheet and get back with further questions (if any). To clarify, we plan to use 2 * Flex radios as receivers only, one unit for 6 * maritime DSC channels, and 1 unit for the voice channels, for TX - 2 * 500 watts Barret 4050/4075 (1 * DSC channel and 1 * voice). Barrett suggests separating RX and TX sites but in our case, it is not possible. Should we add such filter, for safety? https://www.wimo.com/en/12402 Thanks! Leo

KD0RC

The filter website does not give band reject values, so it is hard to say what value these would bring. Are the maritime frequencies you mention in or near the ham bands? If not, then the filter you referenced won't work.

Are any of the receive-only radios listening to the same bands as the Barret transmitters? If so, a band pass filter won't help.

The internal band pass filters in the 6700 are for the ham bands, so unless you are fairly close to a ham band, you won't get the 25 dB benefit that they bring. You will want to keep this in mind as you work through the worksheet.

Mariner2011

Hi, the DSC (Digital Selective Call) frequencies are: 2177.0, 4208.0, 6312.5, 8415.0, 12577.5, and 16805.0 kHz. RX will be constantly monitoring them. TX may transmit on all of them (not too often) and some other maritime voice and aeronautic frequencies for up to 30 Mhz.

KD0RC

Ok, so then the ham radio bandpass filters won't help, and in fact will block desired signals.

When the 6700 is on a non-ham frequency, you will see "WIDE" in the upper right hand side of the SmartSDR screen. (More accurately - if the radio is far enough away from a ham band).

WIDE indicates that the band pass filters are disengaged to allow reception on non-ham frequencies.

I would go ahead with the worksheet to see if your antenna separation and orientation will be OK when using 500 watts. My guess is that you will fine, or if not, that you won't have to move the antennas too much. The front ends on these radios are very well engineered.

Mariner2011

For TX we plan using 2 * Barrett 4050 SDR radios with 500 watt Barrett 4075 line amp and Barrett 917 multi-wire broadband dipole antenna (32 meters if not wrong) installed on 12 meters above ground between two lattice towers. For RX - one or two Flex6700 radios with Barrett 4017 automatic tuning horizontal dipole on the building roof, 6 meters mast. the system intended for Marine Rescue Coordination Center MRCC. Thank you , Leo

KD0RC

The big question is how far apart the antennas are. Plug that into the worksheet to see if your planned antenna locations will work.

Mariner2011

RX will be on the building roof and TX - between two lattice masts 20-25 meters apart from RX antenna. Unfortunately, site has some space constraints…

Mariner2011

Hello! Still waiting for some additional info from site people, to fill out the worksheet. A question: does it make sense to include such or a similar unit: https://mfjenterprises.com/products/mfj-1048 or https://www.herostechnology.co.uk/pages/hf_preselector.html Thanks! Leo

Stan VA7NF

Providing you have an existing TX antenna, or can make a temporary one, transmit low to 100W on or near your target frequency.

Also find a local AM broadcast station with a very strong signal near the low end of the broadcast band.

Now build a table of frequencies that are:

TX freq + and - that strong AM signal

2nd TX freq harmonic + and - that AM frequency, repeat for 3rd harmonic + and -.

While transmitting (and changing the output power) look for those mixing products. Once found you can do portable direction finding to identify the source of mixing products. Fortunately mixing products originate at the point of mixing not at either source frequencies.

You will likely find fencing and rusty towers. Clean these up as they will generate a wide range of signals that will appear as RX noise. Good Luck.

DigitalMariner

Hello @Mariner2011,
I'm also interested in using a Flex6700 as Ham, LandSAR and Marine radio.
How did you get on with this project?

I'm particularly interested in what platform you interfaced with Flexradio to demodulate the DCS msg.
Also, did you interface any third-party bandpass filters to improve the SNR?

Mariner2011

Hello, so far, we have used Flex6700 as RX only in a standalone system, and it perfectly does the job for 6 MF-HF frequencies. We have our own DSC Decoder software (for commercial use), which is installed in 40+ MRCC/SAR sites around the globe. We do not have MF-HF RX and TX on a single site yet, but planning to implement such an arrangement for the customer with space constraints; that's why I was researching the bandpass filters. DSC is very sensitive, and it is recommended to keep its spectrum as clean as possible. The proper way is to separate RX and TX sites 10-20km apart and make sure the minimum of ambient RF noise and other signal disturbance (no common ground with building appliances, AC and so on). VHF (CH 70 for DSC) is much easier; we use it with band-reject filters from Amphenol-Procom (the most suitable but very long lead time).