To Disk or Not to Disk...

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KE5MC

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This is a journey that started a number of years back when I moved from one city to the next one over. Keeping what I was normal scanning and adding to my Favorites List the results were not the best.

In hopes of improving the reception I built what you see “In the Beginning”. Bulkhead female BNC connector on a piece of Prototype Perf board with copper on both sides. Added some 4 inch “spike” radials from stripped Romex. High ceiling room so assembly was placed on top of a piece of PVC with Radio Shack 800MHz scanner antenna and about 8 feet of coax to a 536 scanner. Not much improvement and I think the coax was the issue. Sold the scanner and got a SDS100 and just used the 800MHz with BNC adaptor on scanner. Spike assembly placed on shelf until recently.

Along comes the VNA-Nano analyzer. I got one, played with it and was impressed with what it was for the money and the results. I followed many of the discussions and whole heartedly agree that testing HT/Scanner antenna can be an exercise in futility.

What follows in the pictures is my attempt to create a test stand that meets no known standard of acceptance or certification. Hopefully it does give me something repeatable which is mechanically and electrically stable that allows easy swapping of antennas I have to gage how they perform against each other. The cardboard disk is 12 inches in diameter covered with heavy duty aluminum foil both sides overlapping in the center and outside edge. It only sits on the spike assembly. Spikes have been shortened and looped on the ends to keep from damaging the foil.

I’ve just finished the fabrication and using it with the SDS100 for now. What has happened so far with the SDS100 is the reception has markedly improved for items in my Favorites List I normal scan and I’m familiar with reception. RSSI and NOISE display on the SDS100 are the only measurable indication other than subjective listening to the various agencies in the FLs.

Changes in NOISE are typically lower and fluctuate less. RSSI are a couple of dBm lower. None of the changes significant, but the reception for distant or weak stations “seems” to be improved out of proportion to NOISE and RSSI changes. I have had to reassess my thinking about receiving antennas in this frequency range and the impact they can have on the SDS100.

Future activities will include collecting data using ProScan for distant agencies and comparing with and without the “ground plane” disk. Also using VNA Saver to scan various HT/scanner antennas again with and without the disk.

The journey continues…

Beginning.jpeg

Parts.jpeg

Assembled.jpeg

Installed.jpeg

Under.jpeg
 

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majoco

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So were these just dimensions that you plucked out of thin air or are they related to the frequencies you are interested in?
 

KE5MC

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Picked out of the air. For frequency range of interest I wanted something long enough (4") to have a "possible" positive impact without flopping around. For the disk it was the largest flat piece of cardboard I had and the limited supply of foil was only 12" wide. Version 2 in the pictures is the same 12" disk with 18" heavy duty foil. Strictly a seat of the pants production. Placing it on the SDS100 was more an after thought and I was pleasantly surprised at the improvement, admittedly subjective.
 

N9JIG

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This is basically a ground plane for the antenna and the exact size should not make a whole lot of difference other than "bigger is better". Usually anything above a quarter wavelength in diameter should be effective.

I have had similar results by putting the radio on a cake pan but this should work better as it is more properly positioned at the base of the antenna.

Neat idea!
 

KE5MC

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Yes a ground plane... Interesting the word(s) we choose to use or not. Knowing the technical information and taking the time to document the activities I never used that description/term. I might have been attracted to the 'bright shiny object' and lost sight of being technical. :cool:

Any how, I'm thinking of a way to objectively qualify if it improves reception or not. The seed of an idea using Proscan to collect data on a site that I don't receive well over several hours. Sort the data in the various columns and total the hits per talk group and maybe total the RSSI numbers. Two data sets with and without the ground plane comparing the 'differences'. Something to keep me socially distant... :-(

P.S. Thanks for the like Sheriff.
 

wfiedelman

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I recently purchased a Uniden UBC355CLT from the UK because it provides 6.25 kHz narrow-banding on UHF at a very reasonable price. I monitor only 470-490 MHz and have limited antenna options (no roof antennas permitted in my building). After some experimenting, I created the antenna in the photo using a 6" diameter circular steel plate plus four radials extending out 3" and attached with epoxy to the under-surface of the plate (with good metal-to-metal contact) -- total radius of plate + radial is 6". The antenna is a 6" 1/4-wave NMO magnetic mount Larsen with a 12' RG-58U coax run and BNC connector. The entire antenna sits atop a 12" high plexiglass stand on a window sill facing northwest. I can receive transmissions (mainly NYPD) clearly up to about 12 miles (even transmissions originating from the southeast) despite many large buildings, beyond which the signals start to degrade. Works for me.
 

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