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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 06-05-2010, 11:59 AM
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Unhappy scantenna

I recently put a scantenna on my chimney about 50 feet high from the ground. I used high quality rg 6 coax , about 62 feet long . My reception on vhf lo and uhf is great, but it not good on vhf hi .my handheld scanner with the stock rubber duck recieves vhf hi much better than the scantenna. The scantenna is fed into my old radio shack 2048 triple conversion base scanner. What could my problem be with the vhf hi band?

Thank you , from benwood.
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Old 06-05-2010, 6:40 PM
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Some desense, perhaps? Hard to say - are there many pagers or a FM broadcast station nearby?

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Old 06-06-2010, 10:40 AM
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Thumbs down scantenna

no pagers or strong fm transmitters nearby. the vhf hi signals tend to fade and cutout at times.would cutting the 4 short radials to about 18 inches in length help?
thanks for any advice.
john from benwood,wv.
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Old 06-07-2010, 10:10 AM
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You don't say which (manufacturer) Scantenna your is.

Get hold of the gain graph from the manufacturer - broadband Rx antenna's (which includes Scantenna's) are comprimise antenna's above and below their centre design freq (which is not neccessarily their centre bandwidth frequency). You are almost certainly going to find that this antenna displays higher gain at UHF freq's than it does at VHF freq's, in fact it is possible that the high section of the VHF freq range is actually that section inthe antenna bandwidth that has been sacrificed to obtain unity or better performance across the rest of the design bandwidth.

Once you have the gain/frequency graph you can make an informed decision regards trimming any of the radials - though chances are in my opinion, soldering on extensions (copper coated mild steel or stainless steel welding rods) may well be the way you'll be heading to get improved VHF performance - and that is quite likely to impact UHF performance (such is the nature and comprimise that has to be made with broad band type passive omni-directional antennas).
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Old 06-07-2010, 11:18 AM
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Default scantenna

to benbenrf.
thank you for the info.this antenna was a radio shack clone of the scantenna that i picked up for 5 bucks
on clearence sale 5 years or so ago.i will try trimming the 4 shorter elements to 18 inches.if that does not
help vhf hi does anybody have any ideas for a different base antenna that may improve performance on
vhf hi band?

thanks to all of you.
sincerely,benwood.
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Old 06-07-2010, 1:45 PM
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I have a ST2 on my tower along with an old CM5094A and find the ST2 is very good on VHF high band along with Low band, but UHF is a little poor compared to my old CM. It sounds really odd that your VHF high is so poor, have you tried it on your hand held or another scanner? I would check your elements (fold out & connections) for problems for a rubber duck should no way out do a ST2 if things are right.
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Old 06-08-2010, 2:22 PM
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Default scantenna

bucko,thanks for the info.i did check all connections and connected my hand held to the radio shack
scantenna and results were still poor on vhf hi. vhf lo works great, but not used very much in my area in wv.
may have to try a different base antenna. does anybody know how the firestick rr base antenna performs for scanning the vhf hi band?

thanks to all,from benwood
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Old 06-08-2010, 6:51 PM
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Benwood

Noo ........ don't trim out the radials - at least not yet.

I am not familiar with the Scantenna antenna as a design in its self, but the general generic relationship between antenna element length and frequency (at least with broad band omni-directional type antennas), suggests that lengthening elements is the way to go if improvements in the lower end of the overall antenna coverage is been sort, and that shortening is the way to go if improvements are sort in the upper half of the antenna band width.

I would be starting off with an attempt to establish which part of the Scantenna is responsible for VHF reception e.g. in Discone design it is primarily the length of the downward/outward slanted radials that account for VHF performance. With that principal in mind (i.e. the relationship between frequency and antenna element length) do some research online, or make a few phonecalls to a Scantenna manufacturer to try and establish which part (which radials) on the Scantenna account for VHF reception.

You'll get a lecture about how changing the length of any radial is going to affect overall performance, isn't good for the antenna, blah blah, blah blah ....... but, if thats what you wish to do, and you are comfortable sacrificing performance in the UHF band for improvement in the VHF range, then it should be achievable to some degree.

If you just "want to go for it" so to speak, speaking for myself, I'd start off by lengthing the appropriate radials, say by about 30%-50%.

See now how performance is affected (and make sure you make your comparison/s with the receiver tuned to the same frequency/signal and modulation method used both before & after), and see how UHF reception is impacted (is UHF now sounding any worse or better?).

There will certainly be an effect on antenna resonance, as well as a shift in the antenna design center frequency - at least a measurable change to these characteristics if the experiment is carried out in a chamber, but as I have so often said in my comments: measurable changes in any antenna performance do not neccasserily translate into real world performance/listening changes - which I guess is what is more important for you.

An any event, if there is a VHF reception/listening improvement with the relivant radials extended, then you may wish to try and optimise the improvement by experimenting with different increases in radial length. However, if there is a drop in performance, well then, shortening may well be the way to go, but the chances are that lengthening is going to be the right way to go here (with the right radials lengthened, that is), and last but not least: it's a darn side easier to start off with lengthened radials, and to slowly trim them, then it is to start off with shortened radials, and to keep lengthening them!

Good luck

Last edited by benbenrf; 06-08-2010 at 6:55 PM..
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Old 06-09-2010, 10:09 AM
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to benbenrf.
thanks for your help,i will try to find info on the scantenna and try the ideas that you mentioned.i had a scantenna for years that finally wore out from weathering over about 5 years on my chimney.that antenna worked excellently on all freqs.when weather permits i will climb up and try some of your tricks. ive been scanning since 1975 although i still do not know all the technical things about all of the many types of scanner antennas.

thanks for your help and to the rest of the forum,thanks for your input also

happy scanning,
from benwood.
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Old 06-09-2010, 10:36 AM
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I think you are using a 75ohm coax not 50 ohm and that coud be you problem, but I am going by memory and do not have a cable chart at my office
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Old 06-09-2010, 2:08 PM
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Quote:
Originally Posted by w2smw View Post
I think you are using a 75ohm coax not 50 ohm and that coud be you problem, but I am going by memory and do not have a cable chart at my office
Yes, RG-6 is 75 ohm cable but that is not a problem and the scantenna is frequently sold with RG-6.
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