Bypass Filters for SO2R - Do TX Filters Protect on RX?

Lawrence Libsch

I am a RTTY contester with SO2V experience with another brand radio. I now own a 6600 and have successfully setup and run SO2R RTTY during a contest but had problems when working on 20 and 40M at the same time. I understand the 6600 has 50dB filters on the contest bands, but believe my experience suggests I need bypass filters with my antenna setup. I would like to have the option to run my amp and understand I need high power TX bypass filters, like 4O3A series XL, between the Amp and the antennas to do this. My question is whether the high power TX filters will protect the 6600 on RX? If so, it would seem one set of TX filters would significantly simplifying the number of filters, coax runs and switching hardware required. Will TX filters alone give me the same receiver protection as TX filters plus RX filters?

Larry, K4KGG

Jon_EA2W

Hi Larry,

TX bandpass filters are also RX bandpass filters. The meaning of TX filters is that they are designed to run at the designed power levels without being damaged, so you can run power through them.

RX bandpassfilters will handle very small TX power, so this is why there are bandpass filters rated only for RX.

In my opinion you should look for "low power" (100-200w) bandpass filters as they are cheaper and will have better rejection than the "high power" (1 KW and more) ones.

You need to take into account isolation in everything between your radio and your filters, so the less devices between the external filters and the radio the better. With high power filters you most probably will have an antenna switching system that generaly have poor isolation in 10m and above. This is why low power filters are generally better to avoid issues with isolation as you will only need to put your focus in good quality cable and connectors between the radio and the filters, that´s it.

There is more details to be taken into account, I only tried to explain it in the easiest way I could think of.

For your info, I succesfully run 1,5 KW on a tribander with a triplexer, external low power bandpass filters and a Flex 6600.

Hope it helps.

73!

Mike-VA3MW

@Lawrence Libsch You should be fine with what you have. The good news is the front end of each radio is very very well protected from front end overload. I know the 6600 has higher tolerance and I am checking to see if I can get the numbers for you.

Also, keep in mind that RF can still make it to the ADC front end even if there is no active receive associated with that antenna port. (This is actually true in most HF radios).

This might help from the SmartSDR V3 Manual.

39.4.2 Preamplifier / Attenuator Block

The preamplifier / attenuator block can raise or reduce signals before they are sampled in the ADC block. The overload limit of the SCU is reduced by any gain that is added by the preamplifier. For example, if 10dB of gain is selected using the preamplifier selection under the antenna controls in the panadapter, this 10dB addition must be factored into the overload calculations. The same holds for other amplification levels: if +20dB or +30dB are selected, the respective value must be added to overload calculations. You should only use pre-amplification when necessary and only what is required to raise the noise level in the receiver by 8-10dB over the noise level when the antenna is disconnected.

The preamplifier has protection circuitry to prevent damage above levels of +10dBm. This protection circuitry will cause distortion in received signals if the level exceeds +10dBm. For this reason, pre-amplification should not be used when the input signal level will meet or exceed +10dBm. When performing all testing of a full duplex configuration, it is highly recommended that the preamplifier be disengaged until it can be determined that the power level from the transmitter will not reach +10dBm in any operating scenario.

39.4.3 High-Performance Analog to Digital Converter Block (ADC)

The ADC block, or digitizer, converts the received signals into digital data. All ADCs have overload points and damage points, but there is some variability in overload symptoms. With the FLEX-6000 Signature Series radios, the ADC overload point varies from +7dBm (FLEX-6300) to +9dBm (FLEX-6500 and FLEX-6700). This overload point is a “soft overload” meaning that at this point the receiver will begin to show a performance drop. The ADC generally functions better with increasing signal levels up to this point. At the soft overload point, the receiver will begin to develop spurs that will appear in the panadapter and these spurs will grow as power is increased. A digital overload point will be reached around +12dBm (level varies by receiver) at which point the receiver will cease to function normally, producing substantial distortion in received signals and rendering reception difficult.

At levels above +15dBm (level varies by receiver), the ADC can be damaged so the FLEX-6000 contains circuitry to disengage the ADC from the antenna. While circuitry should protect the radio from a damaging signal, it is highly recommended that station configuration be designed such that signal levels above the soft overload point are prevented from entering the antenna connector of the radio. FlexRadio Systems assumes no responsibility for damage incurred from high signal levels entering the receiver

39.5 FULL DUPLEX ANTENNA ISOLATION REQUIREMENTS

For successful FDX operation, it is essential to understand and plan for sufficient transmit to receive antenna isolation to prevent receiver overload and/or damage. The FLEX-6000 FDX Power Calculation Worksheet can aid in the calculation of antenna isolation and power levels that are suitable for a specific station.

The FLEX-6000 Signature Series transceivers are designed to disconnect the receiver from the antenna when signal levels are +18 dBm to +22 dBm depending on frequency. The front-end protection circuits will begin to engage at approximately +15 dBm. Front-end overload of the SCU will occur with a single tone in the range of +8 dBm with the preamp off. Increasing the preamp gain will lower the overload point by the amount of the added gain.

To provide suitable headroom to prevent SCU overload, we recommend that transmit to received signal strength be limited to a worst case of -20 dBm or lower at the antenna input. With 20 dB of preamp gain, this would provide a single tone input of 0 dBm to the A/D converter leaving about 8 dB of headroom. More isolation is better.

One of the best resources for information on achieving antenna isolation is:

"Managing Interstation Interference, Revised Second Edition" by George Cutsogeorge, W2VJN.

Lawrence Libsch

Jon,

Many thanks for your explanations, and for providing your experience with low pass filters on the 6600.

If you have the additional time, I would like to know what filters you are using and how you are switching them.

Mike,

Thanks for the information and referral to the SSDR manual. I had forgotten that adding preselector gain decreased the SCU overload limit. This may have been a factor in my SO2R problem and I will be attentive to this in the future.

Jon_EA2W

Hi Larry,

I´m using 2 sets of 5B4AGN filters that are not for sale anymore. These are W3NQN type of filters.

Two options that come to my mind are the excellent Low Band Systems filters: https://lowbandsystems.com/collection/contester and OM power https://www.om-power.com/products/om6bpf which are pretty similar to the ones I have. I think both have dealers in the US. THe Low Band Systems filters are without any doubt the best you can find nowadays.

You could control them using BCD or digital outputs coming out from the USB ports of the Flex. See the usb cable interface guide for more information: https://www.flexradio.com/documentation/usb-cable-interface-guide-pdf/

73! Jon

KD0RC

Hi Larry, I would start with this worksheet to see where you stand with your SO2R station. I think it will lead you to an informed decision on what you need to safely operate.

FLEX-6000 Full Duplex Power Calculation Worksheet – FlexRadio

Lawrence Libsch

Len,

Thanks. I've previously run the numbers on those worksheets. More significant to me was the RFI I experienced when working SO2R on 40M TX with 20M in RX that I did not experience when working 40 M with 20 M muted. The worksheet was helpful. It gave me the confidence to run the experiment of operating SO2R barefoot. But one experiment is worth many worksheets.

I want to know if TX bypass filters provide RX protection to the radio on the band for which they are designed. Do you know the answer to that question?

Larry, K4KGG

KD0RC

Hi Larry,

Assuming that you put an external bandpass filter on 20 and one on 40, then yes, they do, but to what degree cannot be said without knowing just how much signal is coupled into the receiving antenna. If the RX and TX antennas are very closely coupled, you may not get away from the problem using high power.

When you operate, the "Wide" annunciator in the upper right hand corner of SmartSDR isn't on, right? If it is, then the 50 dB filters are disengaged (I doubt this is your problem, but I had to ask...).

Lawrence Libsch

Len,

Thanks for your reply.

Yes, I have been very careful to make sure my panadapters are not in "wide" mode. The filters have been active.

It is very helpful to know that the high power TX filters provide RX protection on RX. Low power RX filters spec the amount of blocking in dB. Flex sells the 4O3A high power TX Series XL. What is the blocking spec for the 4O3A filters for RX protection? The sum of the 50dB from the 6600 and the blocking spec for the 4O3A filters for RX would perhaps provide useful information for understanding how well the the 4O3A filters would protect my 6600 at a kW.

Larry, K4KGG

John K3MA

I know you asked about bandpass filter but just in case you had not thought about the poor mans solution. You could add 20 - 30 db of attenuation by using a tuned stub of coax. They work on both TX and RX.

Here is a link to some information on coax, stubs and BP filters you might find useful.

John K3MA

http://audiosystemsgroup.com/Coax-Stubs.pdf

Lawrence Libsch

Jon,

Many thanks for all your shared information. Be well. Good contesting and DXing.

John,

Great article. Now part of my library. Thanks.

Larry, K4KGG