Modulation Help (AM)

[CaptainMitchell]

Hello all.

I've recently started broadcasting just below the 40M band as many others do (WDEF). 

A common theme among the reception reports is that I have a strong carrier but my modulation appears weak. I have to keep modulation 50% or below while transmitting at 100W or else I generate feedback.

Using a MARS-modded IC-7300 for transmit. Audio is fed into the transceiver via the ACC socket. Audio is line level that passes through a 20 dB pad before hitting the IC-7300.

My first thought is to reduce power while increasing modulation. Second thought is that the audio is still too hot. Last thought is that maybe my monitoring radio is just too close to the transmitting dipole (which is outside about 150' away from the radio).

I would like to be able to transmit at full power with as much modulation as possible without the feedback.

Am I asking too much of my setup? Thanks.

Captain Mitchell

[Shortwave_Listener]

I had this problem at one point with an older transmitter. My fix was to ground everything in the setup and keep cable lengths to a minimum, and any excess I still had is bundled up instead of stretched out.

[CaptainMitchell]

I had this problem at one point with an older transmitter. My fix was to ground everything in the setup and keep cable lengths to a minimum, and any excess I still had is bundled up instead of stretched out.

Thanks for the quick reply. Everything is grounded to a SPGP - PC, mixing board, transceiver, power supply, antenna switches and monitoring radio.

[radioartisan]

Tell us about your antenna.  How are you feeding it, and is it resonant? Balun?

[CaptainMitchell]

Tell us about your antenna.  How are you feeding it, and is it resonant? Balun?

Dipole, resonant around 6700 kHz fed with RG-213, terminated with SO-239s. No balun, no RF coming back into the shack.

Weather is finally getting better so I can shorten the dipole to center it around 6800 kHz. Currently, SWR is less than 1.5 (no tuner) on the intended frequencies for that antenna.

[CaptainMitchell]

The more I am reading up on this, appears reducing the power may indeed be the way to go for increased modulation.

ALC is invoked for PEP greater than 100 watts in AM mode on the 7300. Running 100 watts at 100% modulation is 400 watts out and the ALC is close to peaking. Supposedly the ALC is not kind to the output signal when driven near it's limits.

I'm going to mess around with 10-25 watts and 40-100% modulation and see if I get a less distorted, feedback free signal.

Would a rf-sampler or rf-demodulator be more appropiate for looking at the output on my o-scope?

And now understanding the benefits of SSB/FM, 2-100 watts. AM, 1-25 watts.

[ThaDood]

I am assuming that the IC-7300 is running barefoot. Well, are you using a 13.8VDC power supply, that can supply at least 25 amps continuous? Reducing the MAX carrier to 20W will give headroom for 100% MOD. And, if you have extra muffin fans, use them to keep that rig cooler. It's not for 100% duty-cycle of operation. Blow cool air in, but suck-out that hot air out. Hope this INFO helps.

[radioartisan]

Oh, I totally missed you were attempting to run 100 watts AM.  Most every 100 watt HF rig typically can run only 25 watts AM, due to the nature of AM with the carrier and two sidebands.

[CaptainMitchell]

Oh, I totally missed you were attempting to run 100 watts AM.

Yea, dumb me... 

[CaptainMitchell]

I am assuming that the IC-7300 is running barefoot. Well, are you using a 13.8VDC power supply, that can supply at least 25 amps continuous? Reducing the MAX carrier to 20W will give headroom for 100% MOD. And, if you have extra muffin fans, use them to keep that rig cooler. It's not for 100% duty-cycle of operation. Blow cool air in, but suck-out that hot air out. Hope this INFO helps.

Correct, running barefoot with a PS that can supply 32A continous.

Suprisingly, when I was trying to fry my 7300 running AM at 100 watts ( ), it has been running cool. I thought heat would be an issue pushing the duty cycle but so far no issues. ALC doing it's job??

Will do some testing at 20 watts tonight when I get a chance.

Info was helpful, thanks!

[Stretchyman]

100W pep = 25W carrier.

You cant run more than that.

Str.

[Charlie_Dont_Surf]

Yes, as others have noted, 25W carrier maximum for your setup. That's most certainly the primary issue and also the easiest to fix.

However, it also seems that you may be confusing feedback with the distortion that trying to run 100 W carrier and also trying to fully modulate would cause. (Unfortunately, RF feedback can itself create distortion so I can understand some of the confusion.) RF feedback may also be an issue here; I can't tell from what you wrote. You have written, "no RF coming back into the shack" but I beg to differ. There is almost certainly always some RF in the shack, it's just a question of how much and if that level is enough to create issues.

In the same sentence, you wrote "no balun" on the dipole and that's a big mistake. A dipole is balanced antenna, meaning that both arms are meant to be equal and the RF feed also must be balanced. Your coax (RG-213) is not a balanced feed. The way to convert from an unbalanced feed (your existing coax) to the the balanced feed that the antenna expects is with a balun. It is exactly the right tool as the name is a contraction of "balanced" and "unbalanced". I recommend putting a balun at the center of the dipole and feeding it that way. Feeding a balanced antenna with an unbalanced source will likely create issues that will lead to RF in the shack, among other things.

I also always recommend that all the cables entering the transmitter should be wrapped around ferrite toroids as much and with as many turns as possible. Get longer cables if you have to. The toroids should be placed as close as practical to the transmitter along the length of the cable, so, for example, a short cable run from the toroid to the transmitter and a longer run to the audio source or the DC power supply is the ideal result. Check the toroid suppliers specs and get ones that are made for suppression at HF. This may be Mix 31, Mix 43 or maybe Mix 75 but there are other possibilities.

The following image gives you some idea of where I am going. (I'm not affiliated with Palomar Engineers and I am definitely not shilling for them. This is just a convenient photo):

[CaptainMitchell]

In the same sentence, you wrote "no balun" on the dipole and that's a big mistake.

Charlie_Dont_Surf,
My horizontal dipole is constructed using an Alpha Delta C hardware kit https://www.dxengineering.com/parts/alf-delta-c. Currently about 30' in the air but I am going to try to double that over the next few days.

For the current design of the dipole, it seems that this style balun would be useful - https://www.dxengineering.com/parts/rez-cm7. I do need to keep it somewhat stealth, have a very Karen neighbor that loves calling the HOA...  I'm under the assumption that this style balun is just as effective as the 'boxy' ones?

My other antenna project for this week is a DX Commander Rapide. Just completed the footer for the fiberglass post, element cutting is next. Being a vertical, it is an unbalanced antenna and would not requie a balun, correct?

I do have more questions about getting my AM signal out as far as possible but realize it starts with the antenna. Hopeful the low take off angle of the vertical will help. So once these issues are sorted, next step may be a painfully expensive amplifier.
Thanks.

Captain Mitchell

[Charlie_Dont_Surf]

I wrote a whole reply yesterday but HFU crashed as I was editing it and I lost it all. I'll recreate it in brief here.

In the same sentence, you wrote "no balun" on the dipole and that's a big mistake.

Charlie_Dont_Surf,
My horizontal dipole is constructed using an Alpha Delta C hardware kit https://www.dxengineering.com/parts/alf-delta-c. Currently about 30' in the air but I am going to try to double that over the next few days.

For the current design of the dipole, it seems that this style balun would be useful - https://www.dxengineering.com/parts/rez-cm7. I do need to keep it somewhat stealth, have a very Karen neighbor that loves calling the HOA...  I'm under the assumption that this style balun is just as effective as the 'boxy' ones?

People get a bit confused about what constitutes a balun and what constitutes a feedline choke and I think some of the vendors have not helped in that regard.
A feedline choke is not a balun. In any case, an easy way to look at it is that a balun should have a dual leads on one side/end. Here are some images that show what I mean. Though they look similar to the feedline choke, one end has dual leads, circled in red.


Ultimately you want to replace the center portion of your Alpha Delta antenna (the bit with the coax connector) with a balun. You can use of the "boxy" kind or you can use the ones that are shaped like a cylinder. Either way, you will be adding some extra weight to the center of the dipole so you will need either a center support or strong support of the two ends or maybe both.

My other antenna project for this week is a DX Commander Rapide. Just completed the footer for the fiberglass post, element cutting is next. Being a vertical, it is an unbalanced antenna and would not requie a balun, correct?

Correct. This is where the "REZ Antenna Systems CM7 Common Mode Feedline Chokes" (or similar) is exactly what you need. I like the stuff from a company called Balun Designs. Their Model 1115d works well for me for the same purpose. https://www.balundesigns.com/model-1115d-isolation-choking-1-1-balun-1-5-54-mhz-5kw/

I do have more questions about getting my AM signal out as far as possible but realize it starts with the antenna. Hopeful the low take off angle of the vertical will help. So once these issues are sorted, next step may be a painfully expensive amplifier.

Amplifiers are another discussion.

Given you live under an HOA, be especially careful about generating interference with your neighbor(s). Increasing transmit power increases the risk of interference. In "the old days", an HF transmitter would easily get into TVs, telephones, Hifi stereo equipment, etc. With the common use of mobile phones, cable TV, Bluetooth, etc. now, the risk may have lessened, however, there still exists the possibility of interfering with anything that generates audio for someone to listen to, i.e., TV audio outputs, home theater stuff, guitar amplifiers, hearing aids, etc.

Your neighbor might be a good indicator or a "canary in the coalmine": if you don't hear from her and she's one of the closest to you, then chances are everybody else isn't hearing it either.

[RobRich]

Are you doing any audio processing? AGC along with multi-band compression, limiting, clipping, etc. can and typically do squish audio dynamics, but those techniques are what most commercial radio stations do for a higher overall average sound level to keep the transmitter closer to peak wattage outage. Just note way too much dynamic compression can lead to excessive clipping, which can seriously degrade sound quality, so it is a balancing act.

StereoTool is popular audio processor for broadcasting. The free options are typically enough for most hobbyists. There are various presets, including for AM radio, as well.

https://www.thimeo.com/stereo-tool/

As for the transmitter in use, the 7300 has a rather narrow audio filter width. Dropping a quick search reports 100-2900KHz. Fine for spoken content but somewhat narrow for musical content. Some hobbyist make ~3K-wide music sound okay for HF listening but plan on processing the audio to get there. Also you can try ramping up the EQ curve above and below those values to help a little, but YMMV, as I suspect the filter skirts are probably rather sharp on the 7300.