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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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  #21 (permalink)  
Old 08-07-2005, 9:58 PM
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Quote:
Originally Posted by TeRayCodA
Don't use aluminum? It's my main course!

http://hometown.aol.com/teraycoda/myhomepage/photo.html
TeRay:
Very professional looking antennas. Wish I had access to tools to work aluminum like that for a discone. My metal of choice is copper, mainly because I can get plenty of it for free Are you just building teh antennas for GMRS, MURS and scanning or do you work with other radio services?

Slip:
You can build a shorter antenna, but you will need to know the measurements for teh load coil, but the problem there is that the load coil will be lossy compared to just a 1/4 wavelength wire. Reguardless, you will need something to decouple the feedline.
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Old 08-07-2005, 10:36 PM
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Yeah,...No...KF4LNE,It's just a hobby that goes along with my monitoring.I have built so many,there is only so much room on the tower.Some are just in the garage gathering dust.

I played with the GMRS concept a few years ago,but,found some MURS radios at a super low price,so the rest of the story..

The GMRS antennas in the pics have long been stripped/scrapped,parts used for bigger-and-better projects.

-Gotta luv it!
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Old 08-07-2005, 11:32 PM
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Quote:
Originally Posted by SlipNutz15
Also, on that site that says how to build an antenna, it says the thinner the guage the better for reception? Is this correct? Should I use a 1/16 wire or 1/8?
Um, the THICKER the wire and the LOWER the guage makes sense.

The thinner the wire the higher the resistance and the narrower the bandwidth.

Yuck. Nice thick wires, rods and tubes for this lad.

Plec
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Old 08-07-2005, 11:45 PM
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Very nice work, If any of those in the garage happen to be the multi dipole stack or if you decide to make a few more please let me know. I am looking for a good UHF-T antenna. My coverage in that area is sorely lacking.

Thanks
Swest90
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Old 08-08-2005, 10:43 PM
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Hi guys,

I really should pay more attention to these threads, you keep jumping WAY ahead of me! (;->)

A simple groundplane for VHF can be made with an SO-239 and 1/16" dia brazing rod, for UHF 14AWG solid copper wire is stiff enough. For Lo Band the elements get so long you'll have more severe mechanical considerations so I suggest cutting down an old CB groundplane to the proper length. You can't go the SO-239 route with that one no matter what material you use for the elements, that flimsy center pin will snap right off in the slightest breeze.

The beauty of the SO-239 antenna is it needs no fancy mounting considerations. Just shove the coax up through the mast and if you filed the flange flat where the radials are soldered the weight of the coax will firmly seat it straight and level atop the mast.

I haven't seen any remarks about waterproofing, you have to put a gob of silicone sealant on the insulating bushing of the SO-239 or you'll end up with water in the coax. In any case securely double spiral wrap the ENTIRE coax connection with vinyl electrical tape.

Multi-band antennas can get complicated, KF4LNE seems to have that covered fairly well. Maybe heliarc a couple of shorter fanned aluminium rod elements on at the base of the radiator of that CB antenna? Aluminium can be soldered using special solder and flux but a tack weld is much stronger, just ask Mikey. (;->)

BTW KF4, the lengths you gave are fractional wavelengths in free space. The formula for a half wave dipole is 468/F so you can take all others from there. Sorry big fella, you forgot the end effects and velocity factor.
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Old 08-10-2005, 7:59 PM
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Here's a picture of my UHF antenna that is on my roof. It is connected to a metal mounting bracket and screwed into the metal heat duct at the peak of the roof. It is connected to RG-6 dual-shielded satalite CATV cable. and the SO-239 end is soldered on by the local radioshack. It's fed down through the roof to my bedroom and an M connector is use (solderless) on the scanner end. It picks up local (same county and 30 miles away) UHF rather well and also Low-Band (50+ miles away) very well. I was wondering if there is anything else I can do to increase my signal receiving. I understand putting it higher in the air but I can't do that right now. I also have a 12.75" main shaft (1/2 wave correct?) and then 6 - 1/4 wave ground radials. Any recommendations? Also, I have a question about hooking two antennas together. Would it screw up my receiving if I added another antenna to the other end of my house and "T"d the two together and then had a main line running to my scanner?

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Old 08-10-2005, 8:27 PM
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You need to cut the vertical element to 6" or the same length as the radials.
a 1/2 or 5/8 wave long element like you made will not work well without an
impedance matching device at the feedpoint.

You might want to make a combination high band 1/4 wave gound plane and
UHF 1/2 wave J-pole antenna like this:
http://www.arrowantennas.com/gp146.html
http://www.arrowantennas.com/inst/igp146.html

I made one of these with an SO-239 connector and 12 gauge wire and it worked pretty well on the 2m/440 ham bands.
The grounded UHF stub is 1/4 wave long (6 3/8" ) and spaced
1" from the 19 1/4" vertical element.

If you hook two antnennas together with just a T connector you will most likely get poor results. You need to use an expensive multicoupler to do that properly. There are several old posts about that on this site.
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Last edited by nd5y; 08-10-2005 at 8:39 PM..
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Old 08-10-2005, 8:30 PM
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So I need to cut the vertical pole to 6.7"?
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Old 08-10-2005, 8:42 PM
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Yes. The vertical and radials are all the same length on a 1/4 wave
ground plane antenna.
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Old 08-10-2005, 8:44 PM
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Ok. Thank you. To the roof I go!

Can you answer my question about hooking up like another antenna and "T"ing the two together? Would that hurt anything?
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Old 08-10-2005, 8:58 PM
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If you hook several antennas and feedlines together with just a T connector
without isolating them with some type of multicopler or bandpass filters,
the signals from each antenna will arrive at the receiver at different times
(phases) and create a directional pattern which will depend on how long the
feedlines are, how far apart the antennas are from eachother and how
they are oriented to the transmitting station.
Reception will improve in some directions and be nulled out in others.
The opposite is true when transmitting. AM broadcast stations with multiple
towers work this way. Unless you plan the length of the feedlines and
orientation of the antennas to make a directional array, you will have
unpredictable (probably poor) results.
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Last edited by nd5y; 08-10-2005 at 9:08 PM..
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Old 08-11-2005, 12:08 PM
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Quote:
Originally Posted by kb2vxa
BTW KF4, the lengths you gave are fractional wavelengths in free space. The formula for a half wave dipole is 468/F so you can take all others from there. Sorry big fella, you forgot the end effects and velocity factor.
You took the words right out of my mouth...

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Old 08-11-2005, 12:16 PM
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Quote:
Originally Posted by SlipNutz15
Ok, well I found a page that told me how long each antenna needs to be at full length, 1/2 and 1/4 length. Since I can't really have them at full, I'd prolly use 1/4 legth which is what most fire and ambulance rigs use.

My results:
Low band - 20 feet (full length) - 10 feet (1/2 wave) - 5.0 feet (1/4 wave)
VHF band - 6.0 feet (full length) - 3.0 feet (1/2 wave) - 1.5 feet (1/4 wave)
UHF band - 2.0 feet (full length) - 1.5 feet (1/2 wave) - 0.5 feet (1/4 wave)

If any of this information is wrong, please let me know so I may adjust lengths accordingly to band.

As far as the wiring and connectors and whatnot, I still need info about that.
The only one that appears to be "off" is the 1/2 wave for UHF. It should be roughly "1 foot" Vice "1.5 feet". All others are pretty much on for "middle of the band" equations.
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Old 08-11-2005, 1:26 PM
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Low 1/4 = 76.96"
VHF 1/4 = 19.55"
UHF 1/4 = 6.78"

Those are the lengths I've figured out by a little trial and error.
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Old 08-11-2005, 2:12 PM
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Well as stated earlier, there are formulas, but the ones that KF4LNE gave other than the "234/F" are wrong.

These are the ones that you should use:

234 divided by the frequency for 1/4 wave [234/150(MHz)=1.56(feet)*12=18.72(inches)]
468 divided by the frequency for 1/2 wave [468/150(MHz)=3.12(feet)*12=37.44(inches)]
936 divided by the frequency for a full wave [936/150(MHz)=6.24(feet)*12=77.88(inches)]

Just replace the frequency with what ever frequency you want and you'll have the length of the antenna for the wave length you're looking for.
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Old 08-11-2005, 3:55 PM
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Slip - Very nice antenna, looks great. Once you have the antenna working liek you want it you should consider building those to sell on ebay or something. Very nice indeed


My bad on teh math i grabbed the wrong set. But since scanner receiving is not exactly a precision science those would have been close enough to the lengths needed. Now for transmitting, they would have been a bit off but at least they would have been a bit off towards the lower end so they could be trimmed I usually build an antenna using the free space calculations so they are longer than the desired frequency so I have enough extra to trim so i can tune the antenna, better to have too much than too little. As for conencting several antennas together you can use a signal combiner for TV antennas, they usually come in a twin lead and coax style, I use one from the cable company thats rated 1-1500MHz and it works great, no odd noises or anything, but all of my antennas are so close together that if there is any issues coming from radio waves arriving at the different antennas at different times its so slight a difference that my ear cant distinguish it. The only place I have a problem is on a scratchy SO repeater system in another county, during band openings I can hear the 2 simulcast repeaters interfering with each other, but while in that county its as clear as a bell, i guess i get it here from reflections off the mountains and such. I live in a great area for radio use, I live on a hill that has a clear LOS up Watauga Lake and I am at an elevation higher than the top of the lake so its great, I can work simplex for miles to the north east from here and hear radio signals from across the state line in Watauga Co, NC. Its great. My coverage to the south is nice too, not a lot of mountains and I can routinely hear high powered fire dispatches from as far away as the Cleavland, TN area.
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Old 08-11-2005, 10:33 PM
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Quote:
Originally Posted by kf4lne
but all of my antennas are so close together that if there is any issues coming from radio waves arriving at the different antennas at different times its so slight a difference that my ear cant distinguish it.
Your ear won't distinguish a weaker signal from one that would have been stronger if you had only 1 antenna. BTW, "close" on 800 MHz means closer than 3" apart, in which case the antennas will interact directly, rather than indirectly. You can't feed one receiver with 2 antennas without having an interaction between the antennas unless you've invented a new form of physics.
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Old 08-12-2005, 1:20 AM
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Well I wasn't planning on monitoring anything other than 46.060 and the 460 band and I was hoping to use two antennas and have them like twenty feet apart from the signal combiner and the the main line running to the scanner. Would that screw with the scanner's receiving capability that much?

As far as my antenna, I'll get another picture of the revamped one since my vertical pole was too long. I crawled up on the roof in the middle of the night and cut it to size and it seems to be working very week. It even picks up low-band from 60+ miles away pretty damn crystal clear!
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Old 08-12-2005, 9:59 AM
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Quote:
Originally Posted by Al42
Your ear won't distinguish a weaker signal from one that would have been stronger if you had only 1 antenna. BTW, "close" on 800 MHz means closer than 3" apart, in which case the antennas will interact directly, rather than indirectly. You can't feed one receiver with 2 antennas without having an interaction between the antennas unless you've invented a new form of physics.

All of my scanner antennas are mounted on yard arms on my antenna mast and all are about 18 inches out from the mast at the same height, they work great, but yeah, i know you cant feed a receiver with multiple antennas without having those problems but when the antennas are that close together the human ear cant tell that there is a slight diference. As for spacing them 20+ feet apart, then you may start noticing since there are many wavelengths of difference and then the coax losses and such come into play. Basically, no scanner antenna system will ever be great unless you are scanning one band or maybe 2 using a dual band antenna. But in all my years with radio I have never actually been able to hear any difference between a single antenna for one band or multiple antennas for several bands, however I did have a military air band antenna up, it was actually cut low for 240MHz, and it gave my UHF RX all kinds of trouble.
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Old 08-12-2005, 11:11 AM
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Quote:
Originally Posted by kf4lne
yeah, i know you cant feed a receiver with multiple antennas without having those problems but when the antennas are that close together the human ear cant tell that there is a slight diference.
The human ear can't tell the difference, because there is no audio difference - it's a difference in signal strength between what you get and what you would get - which you'll never hear.

Quote:
As for spacing them 20+ feet apart, then you may start noticing since there are many wavelengths of difference and then the coax losses and such come into play.
So does multipath at that spacing for VHF-hi and above.

Quote:
Basically, no scanner antenna system will ever be great unless you are scanning one band or maybe 2 using a dual band antenna.
Or 3 using a 3 band antenna or 4 using a 4 band antenna or ...

Quote:
But in all my years with radio I have never actually been able to hear any difference between a single antenna for one band or multiple antennas for several bands
And you can't detect color with a voltmeter or sound level with a photometer. Using the wrong instrument doesn't prove that there's nothing to measure.
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