Reducing VSWR?

Discussion in 'Mobile Radio Systems' started by KN4WNH, Feb 22, 2021 at 10:58 PM.

ad: L-HROutlet
ad: l-rl
ad: Subscribe
ad: Left-2
ad: L-MFJ
ad: Left-3
ad: QSOToday-1
ad: abrind-2
  1. KN4WNH

    KN4WNH Ham Member QRZ Page

    I got my Diamond CR8900A and mounted it, and it was pretty good out of the box on 2m. I believe it started at 1.8 SWR but as I used it, that dropped to 1.46. It was rainy today and I checked it. 1.66-1.68, which was expected with the rain out there. I figured that, after the rain stopped I would run a copper wire from the mounting bracket to a ground bolt to see if we could lower the SWR. It was fine on 2m but on 10m and 6m it was high.

    I used an old lamp cord for the test. I stripped the ends and clamped one end to the antenna bracket, and the other to the ground-bolt on the frame which the battery goes to. I dropped the 2m SWR reading to 1.63-1.64. Is this normal or am I grounding/bonding incorrectly? My truck is aluminum but I would have assumed at least a 0.1-.05 VSWR drop.

    How should I reduce the VSWR in a more potent way?
     
  2. AI7PM

    AI7PM Ham Member QRZ Page

    You'll not likely get optimal VSWR on a quad-band antenna. The compromises required to be resonant on all four bands means it's not going to be a rock star on any one band.
     
    N1IPU likes this.
  3. W7HV

    W7HV Ham Member QRZ Page

    Generally speaking, the SWRs you're getting a perfectly fine. Lowering them won't do a thing. That's SWR. There are other aspects of antenna performance that will be effected by the grounding...mainly the efficiency and the radiation pattern. A good 50 ohm resistor will have a perfect SWR of 1, but zero radiation. So, rather than tweak things for slightly lower SWR, look where you antenna is mounted, how close the grounding point is (it should be right at the base/mount) and how well adjecent surfaces are bonded to that point. For example, if an antenna is mounted on a fender, the adjacent hood will likely have a poor and distant connection to ground. Adding straps to connect the hood to the fender near the antenna mount can improve things considerably. Here's a pic of an antenna mount I had. Note the gournding straps. There's also a quick connect in that picture for a grounding strap from the front corner of the hood that I connected when the hood was lowered. All four corners of the hood had grounding straps. This was for HF, where it's more important, but it's still important for VHF.
    P3250021.JPG
     
  4. KN4WNH

    KN4WNH Ham Member QRZ Page

    Mine is mounted on the left (drivers) side on the fender. I cannot use my 10m band because the SWR is too high. The lowest part is 2.2 or so, and the ends of the band are around 9. I am looking to improve other bands more than the 2m band, but as I understand it, most modern radios start backing down the power when approaching 2.0 VSWR. I aim for 1.5 or below, but below 2.0 I can live with.
     
  5. N1IPU

    N1IPU Premium Subscriber QRZ Page

    Seconded.
     
  6. KN4WNH

    KN4WNH Ham Member QRZ Page

    I agree. The more bands an antenna does, the less it performs on each. However, I took some pictures to show off the issue. I show off the 2m, 10m, and 70cm bands using my NanoVNA. When I first installed the antenna the 2m band started at 18 VSWR, dipped to 1.1, and then rose and was 1.8 on the high end also. Now it appears as though the center has shifted, which seems odd to me. You will also see that I have the 10m band centered, but the VSWR is far too high to use at all. Granted, I am a technician (studying for general) and cannot use 10m on my radio because of restrictions, but I want it to work for me in the future.

    My idea is to use a ring terminal inserted between the bolt-head and bracket with some 10ga wire run to a good ground location, such as the smaller wire from the negative terminal on the battery where it bolts onto the frame. I could use ring terminals on both ends. DO you believe this would help lower it on the 10m, 6m, and 70cm bands? My 2m repeater is on 146.910, which is a hair over 1.8 VSWR which I can use, but lowering it also would not bug me at all.

    Oh and I measured resistance between the metal bracket and the negative terminal on my battery. It was exactly 12.5 Ohms. Not sure if that is high or not.

    https://imgur.com/gallery/hjGTpFW
     
    Last edited: Feb 24, 2021 at 5:37 PM
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    That don't sound right.

    I ignore SWR unless my radio does not like it. A little high does not mean it will not work.

    A single band antenna can be matched closer and will work better.
     
  8. KN4WNH

    KN4WNH Ham Member QRZ Page

    A single-band antenna will work better, but I have a quad-band radio and limited antenna mounting spots on my truck. I suppose I could try to find a four-way diplexer to split each band out and use four individual antennas (geeky, cool way) but I am not wanting to damage the body on my truck. It's aluminum (harder to get a hole-saw through than steel) and I don't want oxidation or anything.

    I just stepped out and tested bracket to negative battery terminal and I was not seeing the LCD clearly. It stays between 87.6 and 87.8 ohms I do not know if that is acceptable resistance to ground or not. If not, I will buy the 10ga wire and do the grounding I mentioned earlier.
     
  9. AI7PM

    AI7PM Ham Member QRZ Page

    Antenna installs are "damage"? Not if done properly, ie: NMO mount in the metal. A proper NMO install will also not lead to any oxidation issues. As to the aluminum, I've drilled several, using an NMO specific tool, and haven't found them any different than steel.
     
  10. K6EEN

    K6EEN Ham Member QRZ Page

    Seems high. My VHF/UHF NMO mount center-punched in the pickup roof is a tenth or two of an ohm DC continuity to chassis ground. My 3/8x24 stud mount for the HF antennas dangling off the tailgate area is low single digits ohms (like 2 or 3 ohms) DC continuity. Try some ground straps (not lamp cord) bonding the bracket and fender to the body/frame. Hunt around in the engine compartment for ground points and see what they did at the factory for grounding engine components for clues on good points to run your group straps to.
     

Share This Page

ad: M2Ant-1