The KA1RCI Network

The Big Blue Beast - 1995 GMC Suburban

Antenna installations

The first antenna was installed thru the roof in 1995 the week I purchased the Big Blue Beast. It was a Diamond Tri-Band for 144/220/440 connected to the Kenwood TK-742A with a tri-plexer and then as I added new radios for other bands I started using magnetic mount antennas.

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After years of fooling around with several different multi-band antennas on magnetic mounts I finally broken down in 2006 after installing the Troy Communication Console and decided to punch a new hole in the roof for another Diamond multi-band antenna on 52/144/440 for use with the TK-30 series radios.

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Next I decided to break out the 220 MHz module of the TM-742A to a mono band Larsen 5/8th wave. The Diamond tri-band antenna was working on 220 MHz however the performance was poor when compared to other antennas.

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Once I switch the 220 module over to the new 220 5/8th wave the range / performance on 220 MHz in the Beast improved dramatically.

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This image shows the Fall 2007 with a mix of thru the roof and magnetic mount antennas

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Summer 2009 new Larsen NMO installations

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After seeing how much better the 220 MHz 5/8th wave mono-band antenna preformed over the Diamond tri-band antenna I decided to break out all of the radios connected to multi-band antennas with individual mono-band Larsen antennas.

First I started by replacing the small 900 MHz magnetic mount with a new Larsen NMB5T900B which is a 5/8th wave over a 5/8th wave over a 1/4 wave for 902/927 MHz and I also made a custom mount using gold plated NMO mount, double shielded RG-142 feed-line, and a mini UHF connector with a gold center pin. The improvement on my 902 MHz coverage with the MCS 2000 was very noticeable.

Next I replaced the original Diamond tri-band antenna for 144/220/440 that was only being used for 144/440 on the Kenwood TM-742A with a new Larsen 2/70NMO-B dual-band antenna. Again I made a custom mount using gold plated NMO mount, double shielded RG-142 feed-line, and a Type-N connector with a gold center pin. I also replaced the SO-239 connectors on the TM-742A with Type-N connectors.


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With the new NMB5T900B for my MCS 2000 on 900 MHz, and the 2/70NMO installed for the TM-742A 144/440 MHz replacing the old Diamond tri-band antenna in the front of the roof, I removed the second Diamond tri-band for 52/144/440 mounted in the back of the roof and replaced it with a new Larsen 440NMO-B mono band antenna.

This new Larsen 440NMO-B antenna is for the TK-830H used on both the 440 amateur band and GMRS band and was also installed with a custom mount using gold plated NMO mount, double shielded RG-142 feed-line.

Once the 440NMO-B was installed in the old hole form the Diamond tri-band I punched two NEW holes farther back in the roof for the 27NMO-B and 50NMO-B for use with my two TK-630H radios on ten and six meters.

The photos below were taken just after installing the last two antennas for ten and six meters. You can still see the dirt shadows from the magnetic mount antennas and coax runs along with the metal shavings from drilling all those new holes for the NMO mounts.

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This image shows the Summer 2009 with all new thru the roof Larsen NMO antennas

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Then the project really got fun... Using some modified VSWR spreadsheets from my friend John WA1ABI each antenna was methodically swept using a Bird meter.

Kenwood TK-730H - Larsen 5/8th wave

This is the data from the VSWR measurements on the Larsen 5/8th wave mono-band antenna using the Kenwood TK-730H radio.

FREQ Watts Fwd Watts Rev VSWR
143.50 56 4 1.73
144.00 69 3 1.53
144.50 64 2 1.43
145.00 66 1 1.28
145.50 71 0.5 1.18
146.00 79 0.45 1.16
146.50 86 0.92 1.23
147.00 90 2.2 1.37
147.50 85 3.8 1.54
148.00 80 5.4 1.70
148.50 74 6.2 1.81

After the initial plot was completed to find where each antenna had the best VSWR the elements were trimmed and the antenna was swept again until the best VSWR was obtained at the desired frequency.

You can click on these images to see the individual VSWR plot for each antenna.


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Most of the antennas dialed in quickly with only one or two cuts, however a few of them required several cuts before I could achieve the optimal VSWR on the desired frequency. Starting with the original factory length of the whip on the Larsen 220 MHz 5/8th wave antenna the VSWR was 4.44 : 1 with 20 watts FWD and 8 watts REF @ 220 MHz in the center of the amateur band.

FREQ Watts Fwd Watts Rev VSWR
220.00 22 7 3.59
220.50 20 7 3.90
221.00 20 7 3.90
221.50 20 7.2 4.00
222.00 20 8 4.44
222.50 22 8.4 4.23
223.00 22 9 4.55
223.50 22 9.2 4.66
224.00 22 8.2 4.13
224.50 22 9.2 4.66
225.00 22 9.2 4.66

Then after six cuts, and more that 150 individual VSWR measurements, I was able to dial in the VSWR on the 220 MHz antenna to 1.07 : 1 with 18 watts FWD and only 0.022 watts REF @ 220 MHz right in the center of the amateur band.

FREQ Watts Fwd Watts Rev VSWR
220.00 17 0.26 1.28
220.50 17 0.18 1.23
221.00 17 0.12 1.18
221.50 17 0.066 1.13
222.00 18 0.022 1.07
222.50 19 0.044 1.10
223.00 19 0.12 1.17
223.50 19 0.24 1.25
224.00 19 0.4 1.34
224.50 19 0.58 1.42
225.00 18 0.7 1.49

You can click on these six VSWR plots to watch the frequency of the 220 MHz antenna move with each cut in the whip length.

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Copyright 1995-2009 Steven M Hodell

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