Why is my HF tx killing my 800mhz SDR reception

[dimab]

I'm running 2 SDR dongles monitoring an 800mhz system (works just fine).
When I TX FT8 with my HF radio (IC-7300), my SDR dongles lose all reception of the 800mhz system until the HF radio stops tx.
Same behavior goes for 2 regular scanners as well.
The HF tx freq is SO far from 800mhz (40-10m), I don't understand how it could cause this much interference. I have tried TX on the HF radio at 1% power (~1 watt) and still same results.
My HF antenna is an EFHW, feedpoint is 10ft+ away from my scanner antenna (roof mounted discone). My limited understanding is that EFHW can exibit some RF into the shack, but I've mitigated that with grounding and common mode chocke.

What are your thoughts/suggestions?

 

[Lauri-CoyoteFrostbite]

You have an RF saturation issue Dimab. Regardless of the separation in frequencies, RF is getting into everything, and isn't frequency dependent.
Does your end fed antenna have a counter poise? ...these antennas are notorious for feed line radiation... If so, how is it grounded - or is it? A 10 foot separation between antennas's is nothing especially with what must be a barn-door size front end of your SDR's. You need to rethink your HF antenna options.... or perhaps just mute out the scanner when you are operating HF

 

[K5MPH]

Yeah these SDR dont have the greatest front ends,you might want to watch it you could wipe out the Rx on your SDR.......

 

[RFI-EMI-GUY]

SDR needs a preselector filter for 800 MHz or at minimum a high pass filter way above HF.

 

[kv6o]

The typical SDR dongles are "DC to Daylight" receivers. Even if you have them set to receive at 800 MHz, they still see RF energy from above and below. Something local could very well saturate the front end and render it deaf.

You'll need some filtration.

 

[majoco]

As in another post - you need to block radiation above 30MHz from your HF antenna. Most hams including me would put a low-pass filter in their HF antenna feed - these were primarily to stop the dreaded "TVI" - television interference - and are pretty well available anywhere.

Kenwood LF-30A Low Pass Filter

 

[Ubbe]

Same behavior goes for 2 regular scanners as well....I have tried TX on the HF radio at 1% power (~1 watt) and still same results.

Do you have a 10dB attenuator to put on the TX, or use a dummy load? If you have one discone and several receivers you have some sort of active amplifier and splitter solution that might be the problem. No DC power to it when you transmit? Those Stridsberg multicouplers overloads real easy but using 1 watt at HF seems suspicious, but could be like majoco suggest, a lot of the TX power comes out as overtones at multiplets of the frequency. The 7300 can also do 50-70MHz so it probably doesn't have any lowpass pi-filter. Connect the discone coax directly to a receiver and test what happens.

/Ubbe

 

[dimab]

Do you have a 10dB attenuator to put on the TX, or use a dummy load? If you have one discone and several receivers you have some sort of active amplifier and splitter solution that might be the problem. No DC power to it when you transmit? Those Stridsberg multicouplers overloads real easy but using 1 watt at HF seems suspicious, but could be like majoco suggest, a lot of the TX power comes out as overtones at multiplets of the frequency. The 7300 can also do 50-70MHz so it probably doesn't have any lowpass pi-filter. Connect the discone coax directly to a receiver and test what happens.

/Ubbe

oh man. I do have a Stridsberg multicoupler near the receivers, AND a pre-amp where the scanner antenna comes into the house.
The multicoupler is within a few feet of the HF radio and coax. I'll try the discone directly to a scanner! Thanks.
I can't believe how hard it is to find a low band filter

 

[R0am3r]

I don't normally recommend MFJ products, but I have successfully used one of their Low Pass Filters to reduce emissions from my HF station. Take a look at the LP-30 or similar products.

 

[dimab]

I don't normally recommend MFJ products, but I have successfully used one of their Low Pass Filters to reduce emissions from my HF station. Take a look at the LP-30 or similar products.

will take a look.
all of these filters mention SWR... so are they adding some type of SWR figure into the TX line? on top of what the antenna system might already exhibit for SWR?

 

[dimab]

You have an RF saturation issue Dimab. Regardless of the separation in frequencies, RF is getting into everything, and isn't frequency dependent.
Does your end fed antenna have a counter poise? ...these antennas are notorious for feed line radiation... If so, how is it grounded - or is it? A 10 foot separation between antennas's is nothing especially with what must be a barn-door size front end of your SDR's. You need to rethink your HF antenna options.... or perhaps just mute out the scanner when you are operating HF

no counter poise per the manufacturer - it's 20' above the ground.
ground - i have it going through lightning arrestor that is grounded into ground pole.

 

[AK9R]

DX Engineering sells these low-pass filters. They have them with either a 30 or 54 MHz cut-off.

https://lowbandsystems.com/collection/low-pass-filters-1500w

 

[dimab]

DX Engineering sells these low-pass filters. They have them with either a 30 or 54 MHz cut-off.

https://lowbandsystems.com/collection/low-pass-filters-1500w

thank you!
I found it at DXEngineering - Low Band Systems LPF1500-54 Low Band Systems Low Pass Filters | DX Engineering

 

[AK9R]

all of these filters mention SWR... so are they adding some type of SWR figure into the TX line?

They are telling you what the filter does to the SWR of your system. If the filter has an SWR of 1:1 on your frequencies of interest, then the system will hardly know the filter is there (they always have some SWR and some attenuation).

 

[tvengr]

AND a pre-amp where the scanner antenna comes into the house.

Try removing the preamp. Some preamps overload easily and can cause all kinds of intermod problems. I have seen a lot of garbage on the outputs of preamps, especially low cost coaxial in line preamps.

 

[dimab]

Try removing the preamp. Some preamps overload easily and can cause all kinds of intermod problems. I have seen a lot of garbage on the outputs of preamps, especially low cost coaxial in line preamps.

thanks for that! I did try, and still same results.
I removed pre-amp AND multicoupler, still same results. I'll be waiting for the low pass filter to arrive.

 

[prcguy]

A low pass filter will not work, you need a 30MHz high pass. If you don't need to receive any VHF lo band then a high pass with a cutoff just below where you want to monitor will work like a 100Mhz high pass, etc. You could also be getting a lot of RF into USB cables and wrapping them around some #31 mix ferrite will fix that. You would need about 9 turns through a large toroid or maybe 5 or 6 turns through a large snap on type. This is a good idea for any USB powered SDR and can clean up the spectrum quite a bit.

 

[dimab]

A low pass filter will not work, you need a 30MHz high pass. If you don't need to receive any VHF lo band then a high pass with a cutoff just below where you want to monitor will work like a 100Mhz high pass, etc. You could also be getting a lot of RF into USB cables and wrapping them around some #31 mix ferrite will fix that. You would need about 9 turns through a large toroid or maybe 5 or 6 turns through a large snap on type. This is a good idea for any USB powered SDR and can clean up the spectrum quite a bit.

I thought the low pass would stop any TX above 50mhz (this specific filter) and that would cut down on the RF being splattered all over. what am I missing here?

 

[prcguy]

There is very little harmonic energy coming out of a modern HF transceiver and not enough to cause interference in the VHF, UHF and especially 800MHz band. The problem is probably front end overload on the SDR or RF getting into the USB cable and causing problems. I think you will need to find a high pass filter. If you were in this area I could loan you some to try.

I thought the low pass would stop any TX above 50mhz (this specific filter) and that would cut down on the RF being splattered all over. what am I missing here?

 

[Ubbe]

It's hard to believe that 1 watt HF power would make any kind of impact to a scanner or preamp. You are sure you didn't read the power meter incorrectly and it's 1kW ?

/Ubbe