Sunday, December 24, 2017

FM whips

I now have a Sark 110 Antenna Analyzer, and I checked out the Magnum Dynalab ST-2 and the GODAR FM DXR-1000 whip antennas sold for FM.  The Godar is actually sold for all VHF and UHF purposes up to 1000 Mhz.

Both have a VSWR of 10 or so through the middle of the FM band.  This may be about as good as you can do with a skinny whip around this size.  That represents a loss of a few dB compared with a dipole.  An old style rabbit ear dipole, or twinlead folded dipole, has considerably better performance, as I found inside my kitchen over 10 years ago.  I had mounted an ST-2 to the wall, and it was useless compared with the rabbit ears.

But, the advantage of the whip, is you can easily mount it high, higher than anything else because it isn't heavy and doesn't need much support.

And so, it turns out in my "ambitious" new antenna plan, I'm going to mount one of these two whips on the highest point of the fascia board around the roof of the house.  Finally, after a lifetime of wanting to have an outdoor antenna.  I have three other much better outdoor antennas in my collection, but they all require much more ambitious mounting, such as a tower--which is my current long term dream.

But, which one?  Well despite nearly identical performance through the 93-108 Mhz part of the band, the Magnum Dynalab has a notched optimum in the low band between 88-93 Hz.  There it actually has excellent performance, with VWSR dipping sharply to as low as 1.8 at 88 Mhz.  The GODAR has a similar notch, but it's uselessly lower, at 60 Mhz or so when the antenna is fully extended.  Despite what you might think, changing the length has little effect, simply raising the low end VSWR a little, slowly.  It's not like you can tune it in, at least as far as I can see up to 200 Mhz.  The low end notch in the Godar perhaps helps extend the VHF below the FM band, but doesn't help the FM band.

So, if the low FM band is important at all (and to me, it's the best part of all) the ST-2 is the better antenna.  And actually it might even be just a hair better through the rest of the FM band also, except perhaps near the top of the FM band.  (Funny, my hearsay belief had been precisely the opposite about the ST-2, which was why I ventured beyond the mainstream ST-2.)  Many observers believe the solid steel whip construction of the ST-2 looks more durable than the folding style DXR, though the DXR is much stouter and solider than the usual folding whip.  All the better to have the ST-2 at the crown of the roof where it carries the most wind load, and th DXR further down and minimally extended for the scanner 140 Mhz and up.

Now matter how long you extend it, the Godar has an ultimate optimum just below VSWR 10 around 140 Mhz, perfect for scanners, with basically flat response from 100Mhz to 200Mhz otherwise.  Making the Godar as short as as it can go barely affects this at all, except it slowly raises the VWSR below 140 Mhz, becoming significantly worse through the FM band.  It looks to me like the Godar is a nice antenna for my scanner 140 Mhz and above, extended simply to get the desired height, no extension at all might be the "optimum" for scanner purposes except for height.  The GODAR is claimed to be useable to 1000 Mhz, I might keep it fairly short for the best 1000 Mhz performance, which I can't easily measure now.

The antenna generally sold for scanners nowadays is the discone, but any of those is going to require a more serious mount than simply being screwed to the fascia board, as can be done with these skinny whips.

My first priority when I get a serious mount, will be to put up a yagi FM antenna, such as my AP9.



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