Transceiver or antenna issue?

[sparks40]

I built a 2 meter J-pole and hooked it up to my FT-2900 and an SWR/wattmeter for preliminary testing. SWR is at 1.2:1, and i'm reading the correct power on the 5, 10, and 30W settings, but when i increase power to 75W, The wattmeter only reads about 40W. I had the antenna on a 10' pvc pole for testing, and i'm feeding it with a 50' section of RG-58/U, which is all i had at the moment. Any suggestions on what else to check? Thanks.

73,

Dave

 

[ShawnInPaso]

What is the SWR at the 75W setting? If it appears okay, I'd think that either the watt meter is out of alignment of the rig never put out a full 75W to begin with.

I've seen quite a few watt meters that are very inaccurate in this way.

 

[wyomingmedic]

I would certainly agree about the meter. A lot of these VHFers are only good to 50 watts.

Also, what power supply are you using? If it is not able to produce at LEAST 12 volts at the maximum draw, it may be cutting back the power. Put a volt meter at the radio end and key up.

WM

 

[canav844]

I'm assuming the watt meter is a SWR a on a jumper right at the radio and not 50' down the line, or a RF stregth meter for that matter.

What is your power supply? What is the voltage readout on it (full 13.8)? How many amps can it handle High SWR at those p0wer levels, or inferior power supply may cause the radio to not be able to put out the higher draw TX powers

 

[GSPD]

I can't give you a direct answer to your problem, but just suggesting another tool to add for your hobby. It sounds like it's time to invest in a dummy load for testing. These are a great tool in trouble shooting these types of issues. I normally don't buy MFJ stuff, but I do own an inexpensive dry dummy load that's rated for about 300 watts and can be had for about $30.

 

[LtDoc]

I've found that all radios don't always put out their rated power. Not necessarily something wrong with the radio, just misadjusted maybe.
I'd also second the dummy load suggestion. They tend to eliminate several possibilities with the antenna's SWR readings.
This also assumes that the SWR/watt meter is rated for the power levels you want to check.
- 'Doc

 

[K9WG]

Could also be RF coming back down the shield of the COAX. Even though you can have good SWR reading the stray RF can cause the watt meter to read incorrectly. Also the quality of the meter can have an effect on the readings.

 

[fineshot1]

Dave - what make/model watt meter?

Most of the el cheapo watt meters do not give accurate readings the higher in power
you try and test with.

I also third the dummy load need, and a good quality one.

I use a bird model 43 watt meter and a telewave 150W dummy load good to about 2Ghz.

This combo never fails to give me accurate power output readings.

 

[davidgcet]

50' of 58 is going to lose a lot of power AND will cause your meter to read incorrectly. you lose just as much coming back to the meter as you do going to the antenna, so you could have a high SWR and in the right circumstances never see it under low power. you need to test your antenna with a good quality coax run or at least with a short 58 jumper.

as mentioned, check your supply voltage with the radio keyed in high power mode. if you can beg/borrow/steal a dummy load test with it as well.

i would try to borrow another meter just to verify as well, especially if you are reading mid range on the scale. wattmeters are calibrated for the upper end of the scale, mid range and below can be off by 5-10% even on a good meter. this is why many folks that use 43's have a bunch of different elements(i personally have 1/5/10/50/100/500/1000W elements in about 5 different bands) for accurate read.

 

[sparks40]

Dave - what make/model watt meter?

Most of the el cheapo watt meters do not give accurate readings the higher in power
you try and test with.

I also third the dummy load need, and a good quality one.

I use a bird model 43 watt meter and a telewave 150W dummy load good to about 2Ghz.

This combo never fails to give me accurate power output readings.

The meter is a Paradynamics PDC-600. I just wound a choke in the coax (4 turns a 5") right at the feedpoint, and two things happened. The SWR went down to a 1.1:1, and it now gives me a reading of about 60 watts, so i'm thinking the problem was RF coming back on the coax. I'm going to try to tighten the radius of the coils and see if that improves it even more, but i think i'm good to go.

 

[nanZor]

Right on with the coax choke! J-poles are not balanced antennas, and need chokes unless you want the common-mode of the coax to become part of the antenna - so you did the right move there. RS 273-105 snap-on ferrites will work too (about 4 minimum) but are costly.

As for tightening the radius- be careful not to wind too tightly if you are using foam and not solid dielectric, as tight turns can move the center conductor closer to the braid causing problems with foam.

I've been pretty successful by marking out an inch or two less than a calculated quarter wave on the coax with a marker, and then coiling them a few times and making the marks parallel to each other (with turns in between) before applying zip ties. (No need for velocity factor since we're dealing with the outside common-mode current being choked)

Anyway, I'm really glad to hear that the choke brought everything under control.

 

[fineshot1]

The meter is a Paradynamics PDC-600.

I looked over the specs for this meter and its usage looks fine for HF and perhaps
10 meters but above that range I would recomend some watt meters of better quality.

The PDC-600 was calibrated at the factory on 27Mhz per the user manual. That is why
I prefer the Bird Model 43, the elements can be changed to what is needed for each
different freq range of measurement so no need to calibrate, but it does tend to get
expensive with all the elements one can need.

 

[LtDoc]

About meters.
While the Bird line is nice, they are certainly not all that necessary for common use. A lot of that depends on just how 'accurate' you want to be. For general/common use, accuracy just isn't that big'a deal. Of course, it Bird cut their prices in half... I might have one. Until then, I seriously doubt it I run out any buy several.
HF meters tend not to do well on VHF/UHF, so getting a meter that 'fit's it's intended use is a nice idea.
I expect something like 10% accuracy. I have no need of more than that.
- 'Doc

 

[GSPD]

Could also be RF coming back down the shield of the COAX. Even though you can have good SWR reading the stray RF can cause the watt meter to read incorrectly. Also the quality of the meter can have an effect on the readings.

The RF issue is a good point and certainly something to consider. If you left out the step of a choke or balun that will create an issue. It must be placed close to the feed point and about 4-5 turns of coax about 8 inches in diameter. The OP didn't mention a balun in his post.