High SWR Inverted L
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| Jul 26th 2014, 22:46 | |
KC7QHETotal Topics: 0 Total Posts: 0 |
Put this inverted L up with what materials I had available. Have two wooden power poles set 127' apart. They are 55' and 50' above ground at the top. Used #4 stranded copper wire with THHN type insulation still on. The vertical portion goes up 50' and then horizontal for 126'. The ground plane consists of 14 wires, #12 to #8 stretched out 140' to 150', depending on what was available. I have used a MFJ-259 analyzer connected to the base of the pole, no feed line. All across the spectrum 1.7MHZ to 60MHZ the SWR is pegged high. The resistance will drop to 200 ohm at several points but mostly is pegged high. I had this to tune up somewhere in the HF spectrum but no luck. What am I doing wrong. Would appreciate any suggestions |
| Jul 27th 2014, 15:10 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
One would expect a 3rd harmonic resonance around 4MHz. The resistance should be a bit lower, around 100 ohms. The most likely issue is pickup from local broadcast stations. The MFJ 259 uses broadband diode detectors--they have no way of telling these signals from the low level test signal generated by the analyzer. http://www.mfjenterprises.com/Product.php?productid=MFJ-731 MFJ does sell a filter to improve the accuracy of the measurements. I would use an ordinary SWR meter and higher power to tune up the inverted-L. Interference can be minimized by tuning when the bands are closed. Zack Lau W1VT ARRL Senior Lab Engineer |
| Jul 29th 2014, 15:57 | |
KC7QHETotal Topics: 0 Total Posts: 0 |
Thanks. Borrowed RigExpert AA54. It shows a dip at 4.010MHZ I had not though about the background noise. Antenna receives good. Will keep working on it. |
| May 5th 2015, 16:21 | |
0008048069H80Total Topics: 0 Total Posts: 0 |
Would using a 4 to 1 or 6 to 1 BALUN at the antenna feedpoint be advised to make 50 ohms easier for the transceiver match? That would alleviate feedline loss to a degree. Any other ideas? |
| May 6th 2015, 00:29 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
The coax loss can be quite low--LMR-400 is 0.2 dB/100 ft, which is similar to that of window line at 20 meters. It may be a good idea to run some calculations on line loss--your transformer could lose more than you gain. Zack Lau W1VT ARRL Senior Lab Engineer |
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