For years I have had great success with my trusty no-name mag mount 2m/440 mobile antenna. However, it’s time to consider a permanent through-hole NMO mount. NMO product information is a bit misleading so I bought several varieties to figure out the details. I share the results below.
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Irrespective of the fact the solder-the-braid PL-259 connector is probably the worst RF connector conceived by the mind of man1 and the fact the spark shown in the video is in an area that should be a direct short, the video, shown below, wonderfully shows the ease with which static charge can build up on a dipole antenna.
That’s not the point though. If we were to cut off the connector we would surely still see a spark from center conductor to shield. If the cable were near station ground I would expect a spark jump. That large a charge has to go somewhere.
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Over the years I have heard amateur radio folks, CBers and scanner fans bash the use of magnetic mount antennas. Concerns include flying off the vehicle and becoming a projectile during an auto accident. Other concerns relate to performance and stem primarily on how well the shield of the coax is electrically connected to the ground plane of the car.
I cannot comment on the mechanical realities of the antenna flying off the roof during rapid changes in speed, but can say I have never seen one do so. I encourage anyone with data to propose their article to this web site
So…
Let’s talk about the electrical conductivity of a standard mount vs. a magnetic mount antenna. Here is a quote from the newsgroups concerning antennas for scanners…
In the previous post I discuss various attributes concerning the use of Baluns especially in the VHF/UHF bands. I point out other web sites with excellent tutorials on why and where to use Baluns in VHF work. Two particular Balun styles come up strong: 1:1 Coaxial Cable Balun and the Pawsey Stub otherwise known as 1:1 Split Coax Balun. Each Balun’s topologies are shown in Figures 1 and 2. I highlight the mistaken, I believe, assertion one should adjust the Pawsey Stub’s length by the amount of the coaxial cable velocity factor. Figure 3 shows why I think the Pawsey Stub length does not need as drastic a reduction in length as you might do for stubs that contain the electric and magnetic fields within their structure. Finally, I say I will prove my assertion the Pawsey Stub is electrically a 1/4 wave in free space or close.
What follows is the experiment which provides proof Velocity Factor of Coaxial Cables does not apply to cables used simply as a wire in parallel with another wire.
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Numerous folks use baluns for their antenna feedpoints. Toroid chokes are wideband and popular in the HF frequencies.
VHF and UHF antennas, especially beams, often have balanced feed points. Several methods exist to connect unbalanced coax to the dipole element: gamma, t-match, etc. Some VHF/UHF Balun options use tuned stub or shunt topologies to achieve the unbalanced to balanced goal; Methods include the Pawsey Stub, Split Coax Balun and Coaxial Cable Balun. Each of these requires specific lengths of conductors based on multiples of 1/4 wavelength at the design frequency.
When calculating 1/4 wavelengths, however, it seems the amateur radio community has lost the collective knowledge of when Velocity Factor (VF) of Coaxial Cable Dielectric Material applies. Indeed despite the decades of college texts and ARRL Antenna books showing otherwise, the idea Velocity Factor always applies to coaxial cable length calculations when used as nothing more than a wire stub in a Balun seems to have gone viral on the Internet.
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Many folks have successfully modeled all versions of the Hex Beam. Most of the simulation attempts use NEC programs.
Despite being a popular and productive way of simulating amateur radio antennas, another method of electromagnetic simulation makes use of the Finite-Difference Time Domain (FDTD) method.
Without going into too many details, the method divides up the simulation space into little cubes of volume. Then the next magnetic field values are derived from the current electric fields. Then the next electric field values are derived from the current magnetic fields. Repeat.
FDTD makes use of Maxwell’s equations to calculate the above fields. Being a Time Domain method, what follows shows a movie like display of what is happening in the antennas in super slow motion.
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By now I have posted several times about my new G3TXQ Broadband Hex Beam which came online in March 2010. I figure it is time to post a bit more on the materials I used for my version of the hex beam. Read more about the fine spreader kit from Max-Gain Systems.
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This year our club ran 3A for the ARRL Field Day.
Radios included an Icom IC-756 Pro, TenTec Omni VII, Icom IC-746 (for the GOTA) and a Icom 7000 for VHF plus HF spare. All these radios performed quite well despite the high temperatures.
The 100 Watt Elecraft K2 stood above the others not necessarily on performance, but the elegant way the radio’s owner configured his K2, 100 Watt Amplifier and 100 Watt Matcher.
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This post is an all call for any ham radio operators in the Northern Virginia – Maryland – West Virginia area.
Bob Bruninga is organizing the second annual Golden Packet event up and down the Appalachian Trail along with some new West Coast trail activity. Last year the Virginia team was successful in placing an APRS packet repeater on White Rock Cliff. We desire to repeat the process again this year.
Details on the White Rock Cliff ARPS station are on Bob’s site…
Particularly needed is a backup APRS radio, like the Kenwood D700/D710 along with a 7 AH GelCell along with the Hiker to haul it up the mountain.
If you would like to join the hikers in this ham endeavor, please contact John, KX4O, at kx4o@hamradio.me.
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ARRL Field Day 2010 was a blast. A member of our club bagged a satellite contact within two minutes of start time. Wow. One new item to this year’s operation was a G3TXQ Broadband Hex Beam antenna with elements for 20, 15, 10 and 6 meters.
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