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Author Topic: Optimising my HF mobile station - Ununs, reactance, SWR questions  (Read 12129 times)

Posts: 0

« on: July 14, 2014, 10:52:32 PM »

I am gradually 'optimising' my HF mobile station, but would like some advice from the 'experts'.

My screwdriver antenna (Shorty II) is mounted on a substantial metal bar that runs across the rear of my pick-up.

That  bar is bonded to the car body using wide grounding straps, and I've also bonded all doors, hood, engine block and exhaust.

After reading Alan's web-pages (again), I wound the antenna controller power cables around a toroid (ex Zero-Five antenna balun), placed close to the antenna.  That reduced the QRN.

Then I added an Unun (from a Zero-Five 43 foot vertical antenna). to the coaxial feed, again located close to the antenna.

Question - Do I need to use this Unun?  I assume that it will help to minimize any RF flowing on the outside of the coax cable.

Then I started to look at matching the antenna using a shunt inductor.  So the first thing I did was to measure the SWR and antenna impedance for different bands.  I measured at the input of the Unun, after tuning the antenna to each band.  I then tweaked my analyser frequency until the X reactance value was zero, and then read the corresponding SWR and impedance value.

Here is what I got for a few frequencies:

14.250 MHz, X = 0 , R = 40 ohms, SWR = 1.2
18.150 MHz, X = 0, R = 135 ohms, SWR = 2.4
21.250 MHz, x = 0, R = 20 ohms, SWR = 2.3
28.500 MHz, X = 0, R = 69 ohms, SWR = 1.3

Sooooo, I'm trying to understand these results and what they actually mean.  Why does the impedance vary so greatly for different bands where X is zero?

I should add that with my current mobile station, (no shunt inductor yet), I tune for minimum SWR on my meter, (usually 1.0 - 1.3 max), and can then manage QSOs over a good distance (up to 10,000 miles).  But I'm very much aware that tuning for minimum SWR is only the correct procedure if the reactance is also at a minimum at that frequency, which will not happen unless I add and tune a shunt inductor.

Finally, on a general note, my ability to hold mobile QSOs is limited primarily by the QRN from power lines that I'm driving past!  Any suggestions about how (if?) I can reduce that QRN? (without asking the electricity company to switch off power to the town...). Is there a recommended add-on noise blanker that works well to minimize power line 'buzz'?


Posts: 10248


« Reply #1 on: July 15, 2014, 05:42:29 AM »

Go to my web site, and read the Antenna Matching article. You might want to read the Common Mode, and the Choke article too.


Posts: 0

« Reply #2 on: July 15, 2014, 06:36:25 AM »

Go to my web site, and read the Antenna Matching article. You might want to read the Common Mode, and the Choke article too.

Alan, I read them. From my understanding, the Unun may not be a good choice because it will require different tappings to cover all the bands.  Instead, I should be using a 9-turn shunt inductor between antenna feed and ground, and 'tweak' that to get the best compromise input impedance for the bands.

Posts: 0

« Reply #3 on: July 15, 2014, 06:29:39 PM »

Having removed the Unun from the circuit, I am now getting more sensible values on my antenna analyser.  When reactance is zero, I typically get an input impedance between 40 and 60 ohms, on all bands.  The SWR is between 1.0 and 1.2.

One question I have:

When I select a test frequency on my rig, say 14..200 MHz and adjust the antenna controller for a minimum SWR, the actual frequency for minimum SWR that I read off my antenna analyser when measured at the antenna is not the same - if I were to use an electrical half wavelength coaxial cable from rig to antenna, then I would assume that the minimum SWR would occur at the same frequency.

Is it worth adding a shunt inductor at the base of the antenna to aim for a better impedance match? (I am trying to be a perfectionist!)

Posts: 1050

« Reply #4 on: July 17, 2014, 04:18:32 AM »

Rig SWR meters are not very accurate on the whole and if you're using a LED display instead of a needle, even less so. My TS480 for example has three bars to indicate the SWR 1:1 to 1.5:1.

In regards to your antenna input impedance at resonance, you may want to make sure you're doing everything you can in regards to bonding/grounding. 37 Ohms at X=0, the input impedance of a 1/4 wave antenna with a perfect ground, is the target. Bonding is extremely important. I was quite surprised at how much difference just bonding the tailgate of my car to the main body made.
« Last Edit: July 17, 2014, 04:20:50 AM by M6GOM » Logged
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