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Stacy_adams
New member
Username: Stacy_adams
Post Number: 7
Registered: 12-2009
| | Posted on Monday, February 15, 2010 - 12:51 pm: | 
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I have read all the threads about this subject and I have found no definitive answer. Why does SWR rise under modulation? It appears to do so with radios with 'swing'. It is puzzling. Does it rise as power increases? |
Stacy_adams
New member
Username: Stacy_adams
Post Number: 8
Registered: 12-2009
| | Posted on Monday, February 15, 2010 - 6:08 pm: |
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It occured to me after I wrote this question that since SWR must be re-calibrated when amplification is changed, it logically makes sense that SWR will change as the power level of audio increases. Put another way, the SWR is 'out of calibration' from the dead key setting with the increase in PEP.
I think the light bulb just came on and it all makes sense. Duh... |
Stacy_adams
New member
Username: Stacy_adams
Post Number: 9
Registered: 12-2009
| | Posted on Monday, February 15, 2010 - 8:58 pm: |
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I think this is the purpose of dual needle meters - to keep an eye on forward and reflected power relative to SWR. By George, I think I may be getting it! |
Tech833
Intermediate Member
Username: Tech833
Post Number: 142
Registered: 12-2001
| | Posted on Tuesday, February 16, 2010 - 11:24 am: |
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Or....
Your radio with 'swing' may be putting out a lot of spurious emissions and those emissions fall outside the antenna's optimal bandwidth, which would indicate as reflected power on your SWR meter.
The Bird and cross-needle type meters you mention are the best when it comes to this sort of thing.
P.S. Modulation limiters are there for a reason. I have not yet ever heard a radio with a 'swing mod' that sounds good. They may be slightly louder, but more difficult to understand, and the sound is very irritating compared to a correctly modulated signal. Imagine what AM broadcast radio would sound like if broadcasters installed 'swing' modifications. Not good.
Your radio 'Mythbuster' since 1998
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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 10
Registered: 12-2009
| | Posted on Tuesday, February 16, 2010 - 5:52 pm: |
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Thanks, Tech 833. I do have a LP filter after my radio as a precaution and the modulation was checked on a spectrum analyzer when it was professionally set up. I put an 'aaaauudiooo' in and the traces were right at 100%. With normal speech they (positive and negative) average 75 or so. There were several resistors changed as well as a few caps to fine tune the audio both in the radio and in the mic. It has always been known for clear clean audio. This was done in a commercial communications shop many years ago by a colleague of mine while I was present. It was rather involved actually.
I think I'll look into a cross needle meter. I have polled several of my friends and they say their SWR rises a bit. In the grand scheme of things, it is inconsequential but was intriguing nonetheless. I also noted that at the resonance point of each antenna, the forward movement of the meter is nil.
Hopefully this will help those who search for answers to this. I see alot of posts about it.
Thanks again. |
Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 11
Registered: 12-2009
| | Posted on Tuesday, February 16, 2010 - 6:01 pm: |
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As a point of clarification after reading this again, I used 'swing' in the colloguial sense not as in 'swing kit'. I should have used PEP (peak envelope power). My colleague was a AM radio station engineer. I am an avid audio techinican and I have a Symetrix mic pre with compression with a tube limiter before the transmitter to handle the audio input.
Sorry for the ocnfusion. |
