High SWR fix it with an Impedance Match
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- Опубликовано: 6 фев 2025
- Browning BR-78 cb antenna. Could not get a good match with any cb antenna on my 2017 ford transit 350. Had to build a impedance match to fix it. .172uH. Now swr is 1.3.1. which is quite acceptable.
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In conjunction with the loading coil that is on the antenna itself, adding this coil actually creates an L network.
While the built in coil electrically lengths the antenna (hence making it physically shorter when tuned), a coil attached this way has the opposite effect. In a sense they are actively working against each other in regards to antenna length, one trying to lengthen the antenna electrically, and the other trying to shorten the antenna electrically.
Also, for those who can't custom make a coil like this, MFJ actually makes mobile impedance matchers that have multiple inductors, or capacitors if that is what you need, that are attached this way and can be switched between. They are typically used on the large screw driver ham radio antennas because said antennas tend to have impedance that are way off of what is acceptable for some ham bands, specifically the lower frequency bands.
All this being said, it is rare on CB frequencies that you need to do anything more then adjust the antennas length to get a reasonably good SWR match.
Very nifty. I think I will try that to fix my Italy Kl 203p Linear's hatred for a good match.
Your coil will radiate and unwull loose rf signal gain at the coil...done been threw this
My radio is perfect. When my kl-703 is on high its 2.5. Will this work for me. Got a magnet mount stryker
When you use a coil, most of the radio energy is lost and the gain of the antenna is greatly reduced, without a doubt, the gain of the antenna is negative, also this transmission line has high losses, which makes the return current fake.
Eric Muller (Doctorate Rf/Microwwave, Field Communications)
Are you referring to the coil on the antenna or the coil on the coax T?
I have an extra issue. With the linear off the match is fine. This correction needs to work only when the linear is in the on position. I can resolve this by in stalling a new switch inside the linear a dpdt switch allowing the inductance to only operate when the linear is turned on. Next, since I must open the linear's case, I will see if I can install the inductor inside the linear. This however, could dedicate that linear to that band unless there is room for an adjustable inductor.
Did you ever resolve the issue? I have a KL203 and it’s also notorious for harmonics and other spurious emissions.
I recently installed a Sirio 5000 on my truck with a mag mount...I know I know. Still chicken to cut my roof. I added 9ft of coax since the Sirio coax was too short(12ft) and that lowered the SWR while the amp was on. Blame it on loss? Maybe but my meter shows I didn’t lose any power. SWR is around 1.5 with amp on. Pretty happy with that and my TX gets a thumbs up from other operators. I am total noob but just tried different easy options. Try it! Let us know.
@@rallypoint1 Although I have acquired all the things for my lab, I have not had the health to get it operational. There is a RUclips video where a man built an inductor to rectify the problem hope you can find it, it is a fairly easy fix. I just wanted mine to be incased in the unit.
@rallypoint1 I would suggest that the 9ft of coax is acting as a transformer or matching stub. This is because the impedance at the antenna is not 50 ohms but the impedance on the output of the linear is roughly 50 ohms. If those SWR readings are correct, then this is a very good method to impedance match, and the only losses you will incur are those of the coaxial's properties itself (with an SWR of 1:1). Now 1.5:1 is the limit specified by RM for that unit. Going over could damage the output transistors over time, and avoid open circuit or dead short because those mosfets will pop in a split second usually. You could mathematically figure out the exact length of cable required, in theory, and if you are using a mil spec cable or are able to measure it's characteristics accurately with something like a VNA. But like the old adage says, if it ain't broke, don't fix it. And the losses associated with a 1.5:1 SWR are very minimal, it's just the life of the amp if it happens to go higher, mosfets generally 'wear' out from my observations anyway. You've gotta watch you don't overheat the output transformer too, so put a fan on the heatsink. Trust me, I know those units inside out and have owned at least six.
@@arconeagain Hi! That’s a lot of good stuff to take in.
It’s been the same with 2 different antennas and coax that ran from different spots on the SUV. The car is bonded..hood, doors, hatch and exhaust. I unfortunately don’t have a VNA but I still think the amp is the culprit. LOL
The radio (President McKinley) is set at 2w DK with a 11w swing. When amp is on it keys at 25w and swings to around 130w on AM. Your right though it does warm up if I get long winded but there is a fan mounted on top and that keeps it cool.
@@rallypoint1 Okay, interesting. I just bought another two of these, the new ones with the black heatsink, no preamp. I have seen these faulty from new, one with a faulty relay, one had a poorly soldered connection on a switch. One question though, are you placing the swr meter between the radio and the amp, or between the amp and the antenna? I do both as you have to measure the input swr too. You could have a faulty relay. The P model has two, an extra one to switch in the pre amp which I found faulty. Let me know if you're measuring the swr on the output, this could also be a dry solder joint or other component fault.
I got 1.0 on CH-1 and 1.4 on CH-40 I may be able to get 40 even lower thees still some space left on the tuning rings.
Never had any problems with that style of antennas. Don't know what you are doing.
Not nessasary to get low swr...u need to properly isolate rf ground and bond....this will resinate at power outputs causing a loss in field strength
How did you figure out how many turns to use?
I made larger coil with a tap. It had a lot of turns and I used a short wire with an alligator clip. Then just tap up and down the coil till you get a good match. I have an mfj analyzer. So I took the pl-259 of the tee. Measured inductance. Then made a permanent coil. The coil with a tap is fine. When you find your best match. Cut the clip off and solder to that point. More questions? Email me. Sramsdellwdm at g m ai l
I had problems to adjusting my Wilson5000 magnet mount on my Transit. I hat to put it to the middle rear head and cut the whip about 2"... now I’m getting 1:1
And by the way I’m also a Raymond dealer tech down in Montreal Quebec, Canada... 😉
Still a 1.3-1? Dude you have another problem that you didn't solve with that ARRL Antenna Handbook answer. I'm gonna guess that dome mount isn't doing the job, or you're getting a reflection from the other antenna, or you measure the SWR with the van door open. You are being too creative and overlooking a simple mistake somewhere.
Running an FT 891? just get a MFJ 939 tuner and just bolt it to the underside at the feed point and get 15 to 10 with a push of a button..