30-50 mhz antenna?

[prcguy]

Years ago I did an article for Monitoring Times magazine comparing a 9ft military broad band antenna to the Antenna Specialists MON series and an Austin Spectra. On the chart below I used the Shakespeare military antenna as a reference (green trace) and compared the other two antennas with it for precise signal strength using a spectrum analyzer.

If you look at the red trace for the A/S mon antenna you can see it peaks around 35MHz at about -9dB which is using the factory length. At 50MHz the reception is about -32.5dB or about 23.5dB down in receive level compared to the factory tuned 35MHz range. The A/S mon series is not a magic broad band antenna, its tuned to a specific frequency like most and when used out of band the performance drops way off.

 

[prcguy]

But if you want the entire 30-50MHz range one end of it will suffer greatly. All the ones I've tested resonated around 35MHz from the factory and at 30MHz they were degraded some but at 50MHz they loose a lot.

The Antenna Specialists Monitor antennas that covered VHF-Low back in the day were typically cut at the factory for the center of the band. With the antenna covering 25-50 MHz that made the center of the band at 37.5 MHz. If you wanted to optimize performance above 37.5 MHz they included a cutting chart to shorten the antenna for better resonance at the higher frequency that you desired. If you wanted to optimize performance for a frequency below the 37.5 MHz center of the band, you were out-of-luck as you couldn't "add metal" to the antenna's original factory length to make it longer.

I have the instruction manual for the Antenna Specialists MON-52 mobile monitor antenna and it includes the VHF-Low cutting chart. The chart shows the antenna cutting length for frequencies above 37.5 MHz. For frequencies below 37.5 MHz you leave the antenna at its original factory length.

 

[Ubbe]

On the chart below I used the Shakespeare military antenna as a reference (green trace)

Do you know how it compares to a discone over the 100-500MHz range? It seems to be not that bad at upper UHF and there's no competition at LOW-VHF where it really shines.

I guess that a good method to measure frequency response would be to use two antennas of the same model and then reduce all measured values by half, if antennas where available. Then it would show the actual gain it has on its own over the frequency ranges.

/Ubbe

 

[prcguy]

I have never done an A/B test with a Discone, maybe something to consider in the future. I do think the A/S mon series will be slightly better than a Discone at a few specific frequencies in the VHF hi and UHF range.

I'll be range testing a mobile antenna in the next few weeks comparing to other mobile antennas for receive signal strength and hopefully an antenna range test using a log periodic as an exciter antenna sweeping over wide ranges. I'll have all the equipment at hand to do a meaningful test of a Discone over the A/S mon but it will add a few hours to each day of testing. I will consider it.

Do you know how it compares to a discone over the 100-500MHz range? It seems to be not that bad at upper UHF and there's no competition at LOW-VHF where it really shines.

I guess that a good method to measure frequency response would be to use two antennas of the same model and then reduce all measured values by half, if antennas where available. Then it would show the actual gain it has on its own over the frequency ranges.

/Ubbe