Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Optimising my HF mobile station - Ununs, reactance, SWR questions  (Read 2109 times)
HS0ZIB
Member

Posts: 424




Ignore
« 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'?

Simon
Logged
K0BG
Member

Posts: 9879


WWW

Ignore
« 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.
Logged

HS0ZIB
Member

Posts: 424




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

Quote
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.
Logged
HS0ZIB
Member

Posts: 424




Ignore
« 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!)
Logged
M6GOM
Member

Posts: 945




Ignore
« 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
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!