RF feedback and single side band transmissions

Common issue with the Orion seems around (unexpected) RF feedback causing issues with single side band transmissions. When using CW the RF Feedback seems to be of lesser issue but please do monitor its behaviour.

Important to realise is that RF feedback is a combination of sensitivity for RF feedback and actual RF surrounding /entering the Orion. If attention is being paid on how to connect cables to the Orion the sensitivity for RF feedback should be sufficiently low to be RF feedback free. However often the RF signal surrounding / entering the Orion still “exceeds” the Orion sensitivity against RF signals.

First of all the Orion is perfectly shielded from a design point of view. The cabinet is well connected and provides a good shielding against signals going in and out. The challenge starts when cables are connected and RF signals can get access to the RF sensitive parts within the Orion. The microphone cable is a notorious cause of RF signal entry but in essence any cable is providing access.

Allow me to share my current setup as a reference for a bad case scenario

Starting at the antenna being an inverted V with equal length legs (2 x 10 meters). The inverted V is connected to a true balanced tuner using 450 Ohm ladderline. From balanced tuner a length of 1 meter coax cable is used to connect the Orion.

Microphone setup cases;

1- Microphone Heil PR 781, shielded mic cable connected to the front mic connector.

2- Microphone Heil PT 781, shielded mic cable connected to behringer voice optimizer, connected through shielded mic cable to 600:600 ohm audio transformer. All balanced configuration. Other end of the audio transformer connected to Orion DIN connector (unbalanced).

3- Microphone Heil PT 781, shielded mic cable connected to behringer voice optimizer, connected through shielded mic cable to Orion DIN connector. Latter cable in unbalanced mode.

One thing I learned through this setup is that the assumption of the inverted V being “balanced” is incorrect. Its simply not true unless the inverted V is truly symmetrical and is compensated to deal with its surroundings. But which real life dipole is mounted in an open, free and clear surrounding without any objects influencing the dipole balance? Its certainly not the case at my location. What this means I will describe later.

Second is the location of my shack and its in close proximity of the RF radiating antenna. Distance between dipole and radio is around 5 meter and I consider this close proximity. This means chances on RF feedback are a lot higher than other stations where the antenna is mounted on a tower and distance between antenna and shack is more than 5 meters.

As mentioned before two main factors determine success for the Orion

1- Correctly connected and shielded cables making the Orion less sensitive for RF fields:

1.1- Follow the guidelines connecting the microphone to the Orion as mentioned elsewhere on the tentec-wiki page.

1.2- Applying ferrite cores to mic cables could help but in my experience they add only small improvements in most cases.

1.3- The choice to use front panel microphone connector and using the internal microphone amplifier works well despite my bad scenario setup when following point 1.1.

1.4- The choice to use the rear panel din connector and using an external microphone amplifier and 600:600 audio transformer works well despite my bad scenario setup when following point 1.1.


2- Sufficient measures to reduce RF surrounding the Orion and or being able to reach the Orion cables:

2.1- Moving the antenna further away from the radio helps, unless a balanced feedline is used while the dipole is not balanced*.

2.2- Change the antenna system and associated “antenna grounding”. This resulted in the biggest change and making my bad scenario setup to work perfectly well.

2.3- The use of ferrite at the right places will make a difference.

Since changing my antenna system and associated “antenna grounding” was THE change needed let me go into this first. My antenna setup is less than perfect and certainly not perfectly ”balanced”. Using an inverted V with 2 x 10 meter and a balanced feedline sounds ideal however when the dipole is not balanced this means that the common mode rejection is less than ideal. This unbalance results in RF at the balanced tuner which is travelling to the Orion through the coax cable between balanced tuner and the Orion. Where else should it go? I believe this RF leaks into the microphone cable resulting in RF feedback. Am I wrong?

The change I made was textbook grounding

I attached a length of wire (about 10 meters) to the ground lug of the balanced tuner. I changed the coax cable between tuner and Orion to a coax cable with three ferrite cores. Latter would ensure that common mode impedance towards the Orion would be much higher than the impedance towards the length of wire which is connected to the ground lug of the tuner. One could also use a current balun instead. Hence the RF due to unbalance in the antenna system is travelling towards the connected wire instead of the coax towards the Orion.

Perhaps confusing is the RF close proximity case

I did not change the location of the antenna and hence my near field signal did not change. Which confirms to me that the lack of “grounding” was the real issue and actually the Orion RF shielding is working as it should. The RF feedback is not caused by poor shielding or poor microphone amplifier design but is really depending on good stations design. My station design was incorrect.

How to test if your setup suffers from RF feedback?

The easy way is to trust and observe the ALC led indicator and it should follow your voice independent from output power setting. The ALC led responds very quickly to your voice and should follow it blinking happily. To validate for RF feedback here is an easy way to test;

1- Turn down the hardware mic gain to zero.

2- Turn down the output power to 5 watts.

3- Turn down the mic gain to the lowest value possible while monitoring the ALC led following your voice while transmitting.

When the mic gain is set to the correct value and ALC led follows your voice correctly you should not have to change the mic gain anymore. Period.

When increasing power output the ALC led should continue behaving exactly the same as when running low power. If you notice the ALC led behaving differently you most likely experience RF feedback. I also noticed that at the start of transmitting the ALC led behaves normally but fades away when continuing transmitting. It somehow “dies”. The easiest “solution” is to turn up mic gain which “cures” the ALC response but all you are doing is competing with RF feedback. Please do not turn up the mic gain and instead focus on why this happens.

There is an even more extreme validation possible;

4- Turn on the speech processor and set it to level 9.

5- Transmit and if the Orion starts transmitting on its own without you speaking you know something needs to change. In my case the Orion activated ALC without me speaking.

Before and after experience

Before the antenna system setup change the Orion hardware mic gain was set to 10dB and mic gain set to 100. Still my ALC led faded away at times (not always) and it was frustrating. Stations reported my audio sounded raw and to much low frequency audio. I blamed the microphone at that moment. My solution was to move the TX EQ low pass to a higher value and this sounded better according to responding stations. Same experience when using the rear connector, connecting through 600:600 ohm audio transformer, Behringer voice optimizer and the same Heil PR-781 microphone. The result was still raw sounding audio. Stations did not complain but I did not get any compliments for great audio either.

After the change the Orion started to behave like it should according the manual. The hardware mic gain set to 0 db and the mic gain is around 80 now. ALC led is lively following my voice at all times and regardless of output power. Stations report and compliment on audio quality. Speech processor can be set to level 9 and the Orion keeps working like expected. Right now I am using the speech processor at level 6 and it really works well.

My take away’s are:

1- Using front microphone connector is more sensitive for RF feedback however when you get your system configuration correctly it will work very well. Take my bad scenario setup as an example.

2- Using the rear din connector is indeed less sensitive for RF feedback however additional components are needed and in the end all you do is using an external microphone amplifier with perhaps some voice optimisation. Is it worth the money?

3- When your antenna is not perfectly balanced you will have the consequence of the resulting unbalanced common mode signal. Not providing a path to a “ground” means the common mode signal will arrive at the Orion and hence suffer from RF feedback.

3.1- A coax fed dipole with a current balun at the center will force balance between the two legs of the dipole. The effect of the current balun will be a significant common mode rejection and hence small (if any) resulting RF at the opposite side of the coax. Until the dipole is used on other than designed frequencies.

3.2- A balanced open feedline has low loss and great advantages being able to be used on other than designed frequencies. However it assumes a balanced dipole which is only theory. More likely you have an unbalanced system resulting in RF at the other end of the balanced feedline but now with only small loss. In a way this great advantage has also great disadvantages.

4- Always ensure resulting RF at the transmitter can travel somewhere else than your microphone cable, headphone cable and such like. Ensure the path for resulting RF has a very low impedance compared to all your other cables.

4.1- Ferrite chokes will help here to increase impedance towards paths you don’t want the resulting RF signals to travel.

difference between firmware 1 and firmware 2/3

There is no real confirmation available but from experience firmware 1 was less sensitive for RF feedback. The real change has been DSP firmware and the audio processing. I can only guess that somehow the processing delay has changed in combination with the ALC interaction. Perhaps the hang-time changed too. In the end I believe the response to RF feedback is different but it does not change RF feedback taking place.

I am certainly not an expert on this topic. I hope this description will support others when RF feedback occurs with the Ten-tec Orion. Best regards, Onno pa1ap

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