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#1 WRAK968

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Posted 15 October 2018 - 10:17 AM

So, after receiving a few TK880's I threw them onto my meter to test them and ensure they were working. I noticed that on the tower antenna, they were only putting out 5-6 watts on high power (suppose to be 25 watts.) I figured there was an issue with the software on the radio and I went to test mode and turned up the power level, but was surprised when I maxed the level out and it was still only transmitting at 18 watts. So I moved to a smaller antenna and BOOM, the meter now reads 48 watts out. Switch to a base antenna that is in the shop and it reads 28 watts...

 

What the hell lol, I've never seen a radio that the TX power changes depending on what antenna is being used. But I am sure many of you see where I went wrong. Im using an antenna rather than a dummy load for transmitter testing. Still though, I am curious as to why the power fluctuates so much.



#2 n4gix

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Posted 15 October 2018 - 11:48 AM

Have you checked the SWR on  the coax and antenna? Excessive SWR can cause the power amplifier to clamp the output to protect the transistors.



#3 WRAK968

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Posted 15 October 2018 - 01:25 PM

Have you checked the SWR on  the coax and antenna? Excessive SWR can cause the power amplifier to clamp the output to protect the transistors.

1.1-1 on the roof ant, 1.5-1 on the base ant, 1.8-1 on the 1/4wave that read 48W



#4 Elkhunter521

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Posted 16 October 2018 - 10:47 PM

What swr meter are you using? Ive noticed meters that read close to each other on VHF can differ significantly on UHF.
Be vewy vewy quiet.
I'm listening to my wadio!

#5 WRAK968

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Posted 17 October 2018 - 06:15 AM

What swr meter are you using? Ive noticed meters that read close to each other on VHF can differ significantly on UHF.

I am using a Surecom digital meter, The thing is, I am using the same meter on different antennas and getting different results with power out, which makes me wonder if the meter is messed up or if perhaps I am not reading it correctly.



#6 Jones

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Posted 17 October 2018 - 05:45 PM

The reading on a power meter will vary a LOT depending on where the meter sits in the overall total-length of coax cable.  To get an accurate reading, the meter MUST sit at an interval of 1/2 wavelength along the transmission line.  The only way to easily do this is by insuring that you are using a half-wave input jumper cable on your meter.

 

Cable electrical wavelength is figured by the Speed of Light (299,792,458 meters per second) divided by the frequency in Hertz, the dividend of which is multiplied by the velocity factor of the cable (look at the specs for the cable you are using).

 

For instance, if my meter's input jumper was made from RG-213u, I would see that the velocity factor for that cable is 0.66. (sometimes shown as 66%)

 

(NOTE: In this example, I have used 462.600 in the GMRS band as my desired frequency. 462,600,000 Hz)

 

Let's do the math: 299792458 / 462600000 = 0.6480597881539127 * Velocity factor of 0.66 = 0.4277194601815824 Meters.  That is a full-wavelength of cable, and we need a half wavelength, so cut it in half. So, your jumper cable into the meter should be about 21.5 centimeters long, or about 8.5 inches.

 

If you were using a smaller cable, like RG-58u with a foam dielectric, which has a velocity factor of 0.535, then you would need  .1734 Meter jumper, or about 6.8 inch jumper to correctly match the input of your meter.

 

If you are using just some random-length input jumper, particularly if it is over a full wavelength long, then you may not be even close to the half-wave point, depending on the length of the rest of the cable going to the antenna, and the meter will not read right.  If you use this calculated length of input jumper, accounting for its velocity factor, then whatever is on the output side makes no difference to the reading accuracy.

 

By the way, this rule goes for SWR meters as well as power meters.  If you set up your UHF antenna with a 3-foot truck-stop CB jumper going into your meter, then you are likely WAY out of 'whack'. These rules still apply at HF also.  It isn't as critical below 30 MHz since the wavelength is so long, but if you use a 4 or 5 foot long meter jumper at CB frequencies, it WILL be wrong. At 27.205 MHz, use a 6 inch jumper, or else a 9.6 foot one for accuracy.

 

Tech hint: I keep several different lengths of pre-made jumpers in my Bird watt meter case for different frequency bands that I normally work with, and have them all tagged as to what band range they are for.  The higher you go in frequency, the more critical this gets, and the SHORTER they get... sometimes TOO short, thus for 950 MHz, I keep a 15.6 inch jumper of LMR-400, which is 1.5 Wavelengths, but still on a calculated half-wavelength point in the line.  A 5.2 inch long LMR-400 jumper is too short to work with.

 

Long winded, sorry, but I hope this helps.

 

-Jones


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#7 WRAK968

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Posted 27 October 2018 - 05:43 PM

The reading on a power meter will vary a LOT depending on where the meter sits in the overall total-length of coax cable.  To get an accurate reading, the meter MUST sit at an interval of 1/2 wavelength along the transmission line.  The only way to easily do this is by insuring that you are using a half-wave input jumper cable on your meter.

 

Cable electrical wavelength is figured by the Speed of Light (299,792,458 meters per second) divided by the frequency in Hertz, the dividend of which is multiplied by the velocity factor of the cable (look at the specs for the cable you are using).

 

For instance, if my meter's input jumper was made from RG-213u, I would see that the velocity factor for that cable is 0.66. (sometimes shown as 66%)

 

(NOTE: In this example, I have used 462.600 in the GMRS band as my desired frequency. 462,600,000 Hz)

 

Let's do the math: 299792458 / 462600000 = 0.6480597881539127 * Velocity factor of 0.66 = 0.4277194601815824 Meters.  That is a full-wavelength of cable, and we need a half wavelength, so cut it in half. So, your jumper cable into the meter should be about 21.5 centimeters long, or about 8.5 inches.

 

If you were using a smaller cable, like RG-58u with a foam dielectric, which has a velocity factor of 0.535, then you would need  .1734 Meter jumper, or about 6.8 inch jumper to correctly match the input of your meter.

 

If you are using just some random-length input jumper, particularly if it is over a full wavelength long, then you may not be even close to the half-wave point, depending on the length of the rest of the cable going to the antenna, and the meter will not read right.  If you use this calculated length of input jumper, accounting for its velocity factor, then whatever is on the output side makes no difference to the reading accuracy.

 

By the way, this rule goes for SWR meters as well as power meters.  If you set up your UHF antenna with a 3-foot truck-stop CB jumper going into your meter, then you are likely WAY out of 'whack'. These rules still apply at HF also.  It isn't as critical below 30 MHz since the wavelength is so long, but if you use a 4 or 5 foot long meter jumper at CB frequencies, it WILL be wrong. At 27.205 MHz, use a 6 inch jumper, or else a 9.6 foot one for accuracy.

 

Tech hint: I keep several different lengths of pre-made jumpers in my Bird watt meter case for different frequency bands that I normally work with, and have them all tagged as to what band range they are for.  The higher you go in frequency, the more critical this gets, and the SHORTER they get... sometimes TOO short, thus for 950 MHz, I keep a 15.6 inch jumper of LMR-400, which is 1.5 Wavelengths, but still on a calculated half-wavelength point in the line.  A 5.2 inch long LMR-400 jumper is too short to work with.

 

Long winded, sorry, but I hope this helps.

 

-Jones

Not at all Jones. Im still new to how all this stuff works, and being I never had an elmer to teach me some things I've just been doing my best to put it together myself. Ok, so I need to build a shorter jumper as I have a 2' jumper currently in line. Ill look into that.



#8 Jones

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Posted 27 October 2018 - 08:28 PM

Not at all Jones. Im still new to how all this stuff works, and being I never had an elmer to teach me some things I've just been doing my best to put it together myself. Ok, so I need to build a shorter jumper as I have a 2' jumper currently in line. Ill look into that.

 

Glad to help.  If you have any kind of jumper close to 7.5 inches or so, it will likely be close enough at 460MHz.  It may still show a difference in power levels between your different antennas, but not as drastic of a difference as your 2 foot jumper will show.  If you happen to hit it right on the half-wavelength mark, then there should be no noticeable difference between forward power level readings on any of your antennas, unless one of your antennas is bad -  then the transmitter might "fold back", or shut down for protection.

 

ALSO: If you want a true representation of the output power of your radio, don't measure it into an antenna, use a half-wave jumper into your meter, with a 50 Ohm dummy load connected directly to the output of your meter.

 

While I'm here, as long as reflected power is very low compared to forward, or if your SWR is less than 2:1, then it's close enough to work. If it's less than 1.7:1 then it's good. At 1.5:1, it's GREAT! Tweak it if you must, but the difference between 1.5:1 SWR and 1.1:1 SWR is not worth fighting for. In most cases, just follow the manufacturer's cutting chart.  ...and trying to fine-tune a quarter-wave stub at 460MHz is absolutely foolish.


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#9 intermod

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Posted 29 October 2018 - 01:39 PM

The reading on a power meter will vary a LOT depending on where the meter sits in the overall total-length of coax cable.  To get an accurate reading, the meter MUST sit at an interval of 1/2 wavelength along the transmission line.  The only way to easily do this is by insuring that you are using a half-wave input jumper cable on your meter.

 

Cable electrical wavelength is figured by the Speed of Light (299,792,458 meters per second) divided by the frequency in Hertz, the dividend of which is multiplied by the velocity factor of the cable (look at the specs for the cable you are using).

 

For instance, if my meter's input jumper was made from RG-213u, I would see that the velocity factor for that cable is 0.66. (sometimes shown as 66%)

 

(NOTE: In this example, I have used 462.600 in the GMRS band as my desired frequency. 462,600,000 Hz)

 

Let's do the math: 299792458 / 462600000 = 0.6480597881539127 * Velocity factor of 0.66 = 0.4277194601815824 Meters.  That is a full-wavelength of cable, and we need a half wavelength, so cut it in half. So, your jumper cable into the meter should be about 21.5 centimeters long, or about 8.5 inches.

 

If you were using a smaller cable, like RG-58u with a foam dielectric, which has a velocity factor of 0.535, then you would need  .1734 Meter jumper, or about 6.8 inch jumper to correctly match the input of your meter.

 

If you are using just some random-length input jumper, particularly if it is over a full wavelength long, then you may not be even close to the half-wave point, depending on the length of the rest of the cable going to the antenna, and the meter will not read right.  If you use this calculated length of input jumper, accounting for its velocity factor, then whatever is on the output side makes no difference to the reading accuracy.

 

By the way, this rule goes for SWR meters as well as power meters.  If you set up your UHF antenna with a 3-foot truck-stop CB jumper going into your meter, then you are likely WAY out of 'whack'. These rules still apply at HF also.  It isn't as critical below 30 MHz since the wavelength is so long, but if you use a 4 or 5 foot long meter jumper at CB frequencies, it WILL be wrong. At 27.205 MHz, use a 6 inch jumper, or else a 9.6 foot one for accuracy.

 

Tech hint: I keep several different lengths of pre-made jumpers in my Bird watt meter case for different frequency bands that I normally work with, and have them all tagged as to what band range they are for.  The higher you go in frequency, the more critical this gets, and the SHORTER they get... sometimes TOO short, thus for 950 MHz, I keep a 15.6 inch jumper of LMR-400, which is 1.5 Wavelengths, but still on a calculated half-wavelength point in the line.  A 5.2 inch long LMR-400 jumper is too short to work with.

 

Long winded, sorry, but I hope this helps.

 

-Jones

 

Jones:   You may be giving the wrong information here.  In most cases, the length of a transmission line or the jumper between the transmitter and meter will not make any significant difference on the forward or reverse power, unless there is a significant mismatch somewhere in the antenna system.   I never carry any more than one random sized jumper for all measurements from 30 to 1300 MHz.   If there is a mismatch, fix it before continuing.  And always have a good dummy load that is rated to cover your frequency band.   Use the load as the antenna first, then move on from there.    Good RF loads have never been lower in cost than they are today.  

 

It appears that there is a mismatch, and I agree that the power amp may be backing off.  When a mismatch exists, the length of the cables will matter because of standing waves on the line.   If you are forced to use a mismatched antenna system, then you can custom cut a cable length to measure it, then another to present a good match to the transmitter using the antenna. 

 

Greg


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#10 Jones

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Posted 30 October 2018 - 09:34 AM

Jones:   You may be giving the wrong information here.  In most cases, the length of a transmission line or the jumper between the transmitter and meter will not make any significant difference on the forward or reverse power, unless there is a significant mismatch somewhere in the antenna system.   I never carry any more than one random sized jumper for all measurements from 30 to 1300 MHz.   If there is a mismatch, fix it before continuing.  And always have a good dummy load that is rated to cover your frequency band.   Use the load as the antenna first, then move on from there.    Good RF loads have never been lower in cost than they are today.  

 

It appears that there is a mismatch, and I agree that the power amp may be backing off.  When a mismatch exists, the length of the cables will matter because of standing waves on the line.   If you are forced to use a mismatched antenna system, then you can custom cut a cable length to measure it, then another to present a good match to the transmitter using the antenna. 

 

Greg

 

Greg,

 

I fully agree with this, and I'll admit that I was trying to simplify something that just isn't so simple.  If the antenna has a high SWR, a wattmeter may not read correctly anyway, due to the reflected voltage not necessarily being at a half-wave point in the return path.

 

The main problem is that many consumer-grade RF power meters do not actually measure power.  They measure a sample voltage at a given point in the transmission line, and estimate power based on a low-voltage/high-current place on the line.  Reflected power, depending on the phase and time of arrival at the metering point will either add to, or subtract from, that representative voltage, giving a false reading.  For example, if your input jumper is too long, and falls at a voltage high point in the overall line length, and the reflected power from a mismatched antenna also falls at a voltage high at that same point in the line, your voltage-driven power meter may show 45 Watts output from a 25 Watt transmitter.  With a half-wave input jumper, you will start out with a known voltage low point. A mismatched load will still cause a false reading, but usually not as severe. - Your reading will be "less wrong".

 

I guess in summary, if you want to measure output power of your transmitter, use a known good terminating resistor - A.K.A. Dummy Load. I've seen good 50 Ohm terminators selling for less than $40.


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#11 intermod

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Posted 30 October 2018 - 05:19 PM

Yea - I think we are saying the same thing.  After writing that I realized that the consumer meters would likely be voltage sensing.   I have never used one of those so I can see where the issue might come up.   Agree on the loads - and I just sold a relatively new Bird watt meter for $150 - these have dropped in price dramatically as well. 

 

However, on a relative basis, test gear are still much more expensive then the gear they are intended to measure. 

 

G    



#12 WRAK968

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Posted 30 October 2018 - 06:34 PM

Greg and Jones, If it helps, the SWR on the antenna, tested by both an MFJ analyzer and the surecom meter, shows a 1.2-1.3 SWR reading (Depending on if I am sweeping the 462 or 467 frequencies) so SWR feedback shouldn't be the problem when talking about power output. I suppose there could be an issue with the antenna itself but I would think if that was true that the repeater would back down power, which it doesn't now that I have a good duplexer. That's why I was at a loss. I do have another question though. When making a jumper, should I also include the 4-6" length of the TK880's antenna pigtail in my jumper measurement or do I start my measurement at the connection?



#13 Jones

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Posted 31 October 2018 - 09:59 AM

Mr. Ayers,

 

You said you were using a Surecom digital meter. The one I found (SW-102) is definitely a voltage sensing type meter.

 

The website also says (in very poorly translated ChineEnglish, with color photos) that the Power reading will not be accurate without using a 50 Ohm terminator.

 

Also, you mentioned duplexer - that will also show a false reading if you go between the radio and the duplexer. Are you making a repeater? ...or is this a VHF-UHF type duplexer on a dual band antenna? (I don't recommend dual-band antennas. They are a compromise on both bands.)

 

I am still thinking you might have a bad antenna, or at least one that is mis-matched enough that there will be no way for you to get any kind of an accurate reading on that meter.

 

I also must question the accuracy of these Surecom meters in general.  On their website, they show a very clear photo of a forward power level of 7.59 W with a Reflected power of 0.818 W, and show the SWR as 1.24.  If the power levels were accurate as shown, the SWR would actually be 1.98 - Bad.  In another example, they show a forward of 6.75 W, and reflected of 0.072, with an SWR of 1.02. Again, if the power levels were correct, this example would calculate to 1.23, not 1.02.

 

I would suggest taking these readings with a grain of salt.  The meter will show you if it is really good, or really bad, but accuracy does not seem to be there.

 

Just for curiosity, what does your forward power, reverse or reflected power, and SWR read on that meter?  .or do you have the model that reads both forward and reverse/reflected power?






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