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Thread: SWR Good but Output wattage drops with frequency. Normal, bad meter, or loss?

  1. #1

    Default SWR Good but Output wattage drops with frequency. Normal, bad meter, or loss?

    New Tech, KI7JLK, here with a question:

    I have an 8w HT and a copper cactus cut to 147.360 lengths. I used the online j-pole calculator. It's fed with 50' of LMR-400 via two short jumpers (RG-174 and RG-58) with adapters to the HT. and Nissei RS-40 single needle SWR/Power meter. The antenna leg is mounted in an ABS plastic plumbing vent on my roof with a metal stove pipe 8' away. I coiled the 400 into four 10" loops in the form of a large balun, though, from what I've read, a balun may not be needed and my iteration of it is likely ineffective. I hit repeaters 17-20+ miles out with full quieting at 70cm., a bit less at VHF.

    The meter indicates a 1.1:1 SWR plus or minus a tenth on the 2 meter freqs, but the power varies from 4.2 to 5 watts across the band. On 70 cm. the SWR indication is 1.5 plus or minus .1 but the power is 10-11 watts. The meter has questionable reviews. A 6:1 SWR would cut my power in half. It makes sense that the meter could be bad.

    Should the output remain fairly constant at a low SWR across the bands, and therefore my meter is lying to me?
    Do I need to make a better balun out of RG-58 wrapped around a 1 1/2" pipe to stop RF from coming back down the line, (increasing SWR?)


    Opinions needed.

    Don

  2. #2
    GTGallop's Avatar
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    The J-Pole isn't a true Dual-Band antenna. It is a two meter antenna that sorta doesn't suck on 70 centimeters so expect GOOD SWR on 2 and Meh on 70.

    Something else to consider.... For some reason, many radios do not transmit at full power on 70cm. I have seen HT's that were 5w on 2, but 4w on 70. The reason I am told is because 70cm, being a shorter radio wave, travels further on the same wattage. Not sure I buy that because, then why does my radio in the car do more than 4w on 70 cm? It does 50w on 2 but only 40w on 70. Still a step down, but scads more wattage than the HT.

    What brand / model is your HT? I assume not a Baofeng because you bought 50' of LMR-400 (aka The GOOD Sh...tuff).
    KG7NDC

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    Quote Originally Posted by GTGallop View Post
    The J-Pole isn't a true Dual-Band antenna. It is a two meter antenna that sorta doesn't suck on 70 centimeters so expect GOOD SWR on 2 and Meh on 70.

    Something else to consider.... For some reason, many radios do not transmit at full power on 70cm. I have seen HT's that were 5w on 2, but 4w on 70. The reason I am told is because 70cm, being a shorter radio wave, travels further on the same wattage. Not sure I buy that because, then why does my radio in the car do more than 4w on 70 cm? It does 50w on 2 but only 40w on 70. Still a step down, but scads more wattage than the HT.

    What brand / model is your HT? I assume not a Baofeng because you bought 50' of LMR-400 (aka The GOOD Sh...tuff).
    GT,

    What you're forgetting about RF output levels on 2 m & 70 cm is the fact that the manufactures have to deal with the Frequency on UHF versus VHF 2m & the RF transistors Parameters. 70 cm is three times as high in Frequency as 2 m is so most manufactures specify a lower RF output level on UHF / 70 cm. To increase the RF output level at 50 W then the RF transistor would as least double the cost for the additional 10 w or to increase the RF output level to notice a difference in your signal you would need to increase the RF output by +6 dB or 4 times the 40 W to 160 W, the same holds true for 2 m. These rules hold true for Handheld transceivers too. I use the Baofeng Handheld as an example, the UV-5R 5 W 2 m would need to increase to 20 W on 2 m and 4 W 70 cm would need to increase to 16 W 70 cm to provide a +6 dB or 4 times the RF output level to notice a difference at the receiving station's end.

    All of these Homebrew J-Pole Antennas are used for Dual Band 2m / 70 cm:

    https://www.google.com/search?hl=en&..._hk4SiySV9M%3A

    Dan
    WA9WVX

  4. #4

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    Quote Originally Posted by drclaypool View Post
    New Tech, KI7JLK, here with a question:

    I have an 8w HT and a copper cactus cut to 147.360 lengths. I used the online j-pole calculator. It's fed with 50' of LMR-400 via two short jumpers (RG-174 and RG-58) with adapters to the HT. and Nissei RS-40 single needle SWR/Power meter. The antenna leg is mounted in an ABS plastic plumbing vent on my roof with a metal stove pipe 8' away. I coiled the 400 into four 10" loops in the form of a large balun, though, from what I've read, a balun may not be needed and my iteration of it is likely ineffective. I hit repeaters 17-20+ miles out with full quieting at 70cm., a bit less at VHF.

    The meter indicates a 1.1:1 SWR plus or minus a tenth on the 2 meter freqs, but the power varies from 4.2 to 5 watts across the band. On 70 cm. the SWR indication is 1.5 plus or minus .1 but the power is 10-11 watts. The meter has questionable reviews. A 6:1 SWR would cut my power in half. It makes sense that the meter could be bad.

    Should the output remain fairly constant at a low SWR across the bands, and therefore my meter is lying to me?
    Do I need to make a better balun out of RG-58 wrapped around a 1 1/2" pipe to stop RF from coming back down the line, (increasing SWR?)


    Opinions needed.

    Don
    Hello Don,

    Most likely the VSWR (Voltage to Standing Wave Ratio) across the 2 m does change causing your VSWR / RF Wattmeter to read erroneous RF Power readings but I've never built or owned a homebrew J-Pole. The ONLY method to prove this problem out would be to disconnect the Antenna and place a known good 50 Ohm RF Dummy Load on the Output of the RF Wattmeter and then perform the same test. I'm not sure why you tuned your J-Pole for 147.360 MHz when you have privileges 144.5 MHz through 148 MHz for Analog FM operation. 144.000 MHz to 144.100 MHz is set aside for CW ONLY, 144.100 MHz through 144.500 MHz is set aside for Weak Signal operations, i.e. CW, SSB, AM, PSK-31, JT-65 & JT-9 modes. In fact, here's the ARRLs Recommended Band Plans: http://www.arrl.org/band-plan Now scroll down to look at 2 m & 70 cm. Since you're a Technician you have privileges on 10 m, 6 m, 2 m, 1.25 m, 70 cm, 33 cm, 23 cm and the Microwave Bands.

    Dan
    WA9WVX

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    GTGallop's Avatar
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    Quote Originally Posted by WA9WVX View Post
    What you're forgetting about RF output levels on 2 m & 70 cm is the fact that the manufactures have to deal with the Frequency on UHF versus VHF 2m & the RF transistors Parameters. 70 cm is three times as high in Frequency as 2 m is so most manufactures specify a lower RF output level on UHF / 70 cm.
    AH HA! Now that's an explanation I can understand!
    Thank you!
    KG7NDC

  6. #6

    Default "GOOD STUFF"

    What brand / model is your HT? I assume not a Baofeng because you bought 50' of LMR-400 (aka The GOOD Sh...tuff).

    Thanks for the chuckle! I'll confess to the ridiculous notion of installing good cable for a lowly Baofeng. I read it was wise to invest in the antenna side of your system, so I did. With the effort involved in installing the coax in the wall, eating a lot of insulation while nearly getting stuck in the narrow truss width between vaulted ceiling and roof, I couldn't justify RG-58. Now I need a real antenna and radio!

    Yes, the little Baofeng tri-power has twice the power output at 70 cm than 2 meter, with the slight penalty of higher SWR, provided the meter is reading correctly.

  7. #7

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    I tuned to the local repeater used for the local net and emergency, and positioned the antenna stub toward the repeater for whatever gain might be offered. It's 17.5 miles out over a couple big hills, and I can hit it
    I also have a GMRS ticket so the wife can rave at me long distance, so I tuned for the higher freq. hoping to get better performance at 462 mhz. Thanks for the tip on the dummy load. I can get a cheap 5watt for the test. Yes, HF, digital, and all that other stuff sounds very interesting! A decent base station and mobile rig may become a reality before long.

    Thanks for the replies, Gentlemen
    73
    KI7JLK
    WQYR435

  8. #8

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    1.5? Most meters rely on two diodes to display the forward and reflected power. They rarely are a perfect match, and as such, the VSWR appears to drift a little. This is why really accurate meters use a single element that is then rotated to read the reflected VSWR. However - 1.1 to 1.5 is a very small amount of wasted power, even if it's correct. Me personally - on my meter, anything less than 2 I could live with, and 1.5 would be great. My reasoning being that to detect real differences below 2:1 needs a very good meter. One of my aerials reads a tad over 1.5:1 on my meter, but a friends Bird Thruline says it is 1.25:1. I believe the Bird. You could just be chasing your tail.

    The key question is - what is the performance like? Can you hear what you expect to hear? If you can - move on and enjoy the hobby.

  9. #9

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    Quote Originally Posted by paulears View Post
    1.5? Most meters rely on two diodes to display the forward and reflected power. They rarely are a perfect match, and as such, the VSWR appears to drift a little. This is why really accurate meters use a single element that is then rotated to read the reflected VSWR. However - 1.1 to 1.5 is a very small amount of wasted power, even if it's correct. Me personally - on my meter, anything less than 2 I could live with, and 1.5 would be great. My reasoning being that to detect real differences below 2:1 needs a very good meter. One of my aerials reads a tad over 1.5:1 on my meter, but a friends Bird Thruline says it is 1.25:1. I believe the Bird. You could just be chasing your tail.

    The key question is - what is the performance like? Can you hear what you expect to hear? If you can - move on and enjoy the hobby.

    I just find it hard to believe i built the "perfect antenna" with low swr across all ranges of both bands including GMRS, given the reputation of J-poles. This is my first real antenna build, outside of a couple of 300 ohm twinlead experiments. I thought the reduced power in 2 meter freqs was a result of a high swr that was not indicated on the possibly defective meter. Dummy load on order. Signal reports are good, I'm happy, but always curious and wanting to learn more.

    It's also ungrounded and I want a ground for lightning protection, but research shows it may act as a radiator and degrade the antenna. My Florida scanner antenna was hit once with very interesting results. All new appliances and my dog was never the same.

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