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Dipole, Off-Center, Balun, No Balun, 50 or 75 Ohm Coax?

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2guntom

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#1
Just started playing with the home-brew antennas for my scanner family. I have 2 I am using now, plus 4 purchased antennas...

I've been reading, and reading, and looking at pictures, studying designs...

... and there's a few things that seem unclear to me:

1) for dipoles, real dipoles, center-fed, do you use 50 ohm or 75 ohm coax?

2) For center-fed dipoles do you use a balun?

3) For center-fed dipoles, do you used a balun ONLY if you used 75 ohm coax?

Again, this is for scanner use.

Also, just for listening, is the length of a center-fed dipole critical?

For instance, if a 36" dipole (2x 18") works well for 154 MHz and 440-470MHz, if the measurements were increased to 10' (2x 5'), will it bring in the signals better because there is more metal in the air?

prcguy mentioned in another thread that 1/2 wave is bad, but that was "end-fed". I believe "odd multiples" of a 1/4 wave were good, so would 2x 54" legs be better since they are 3x 18"?

I've got results from several antennas already, but I don't quite understand the results, or at least I don't want to draw conclusions from assumptions. I'll leave this for a later post till these basic questions are answered.

Thanks in advance :)
 

LIScanner101

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#2
For a regular dipole with equal "pole" lengths the antenna becomes resonant only on the lowest frequency it's cut for. In the case of a 10' dipole (2 5' sections) it would be resonant on a VHF low band frequency. The idea of the OCFD is to increase its bandwidth but it winds up being primarily resonant at about 85MHz which doesn't do much for scannists interested in the 30-50MHz public safety bands.
 
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#3
A half wave overall length center fed dipole at a specific frequency used vertically is a good starting point for making or comparing scanner antennas and it will have an impedance close to 72 ohms but using 50 ohm coax is not a big deal. A 1:1 balun at the feedpoint can isolate the antenna from the coax some and reduce feedline radiation but it’s not mandatory.

This vertical dipole will have a mostly omni directional radiation pattern at the horizon and you would bring the coax away from it at a right angle as far as possible to avoid disrupting the pattern. The band width will be relatively narrow and if made for 155MHz as an example, it will start degrading below maybe 150MHz and above 160Mhz if made from thin wire. Making the elements fatter will increase the BW but there are some limits.

At 2X the design frequency or 310Mhz for our example, a dipole will be very high impedance at the feed point and not very useful for feeding 50 or 75 ohm coax. The pattern will have shifted from a main lobe at the horizon to two main lobes pointing up and down at around 45dge with a null or loss of reception at the horizon.

At 3X the design frequency, or 465Mhz for our example, a dipole will have a reasonable match to coax again but the radiation pattern will favor up and down at roughly 45deg angles with some energy at the horizon but degraded from its fundamental frequency pattern. This is a problem when making vertical multiband antennas for VHF/UHF where on HF, multiband dipoles are used mostly horizontal and the antenna can be oriented to make good use of the gain lobes.

The above described antenna would not need a balun for its fundamental and 3X mode but you should be more specific when speaking of baluns because some are used to strip RF from running down the coax without transforming impedance (a 1:1) and some are used specifically as a transformer to change impedance like matching a 200 ohm antenna to 50 ohm coax would use a 4:1 transformer and the word balun is an acronym for BALunced to UNBalanced, which is another function of most transformers.

Hopefully this answered some of the OPs questions but a Google search will give you endless information on antenna patterns at different wavelengths, impedance expectations and balun information. Antennas are very complicated and I find the more I learn, the more I realize I don’t know, but that makes me want to learn more and hopefully this forum sparks a similar interest for others.

My opinion is, making a multiband antenna that has an acceptable pattern at the horizon and has a good match to 50 or 75 ohm coax can be a challenge. If successful it will probably be fairly narrow band at each frequency its designed for, assuming you have ways to test and evaluate its performance. If you want consistent and predictable performance over a very large frequency range then you’re probably better off with a Discone.

You want multiple bands and gain over a dipole you say? You might look into separate high gain antennas for each band and a di-tri-quad-etc-plexer to combine them with minimal loss.
prcguy
 

davenlr

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#4
You can also combine several 1/2 wave dipoles at the feedpoint, one each for each band you are wanting to listen to. Keep the wires inside a PVC pipe, and separated from each other as much as possible. Each resonant dipole will pick up the band its cut for, and the others will be relatively ineffective. I have used this type dipole on HF, where I had 10, 15, 20, and 40 meter dipoles all connected to one coax. It worked quite well. You could experiment with it, and see how it works. Its basically the same theory they use for the ST2 scanner antenna.
 
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#5
You have to be careful with odd multiples like a 40m dipole will have a low impedance on 15m and if you parallel a 15m dipole with a 40m the RF current will flow on both dipoles and the feedpoint impedance will be about 1/2 of one dipole.

Even multiples are usually fine like a 40m and 20m dipole together where the 20m is essentially out of the circuit on 40m except for some added capacitance between elements and to ground from the extra wire.
prcguy

You can also combine several 1/2 wave dipoles at the feedpoint, one each for each band you are wanting to listen to. Keep the wires inside a PVC pipe, and separated from each other as much as possible. Each resonant dipole will pick up the band its cut for, and the others will be relatively ineffective. I have used this type dipole on HF, where I had 10, 15, 20, and 40 meter dipoles all connected to one coax. It worked quite well. You could experiment with it, and see how it works. Its basically the same theory they use for the ST2 scanner antenna.
 

LIScanner101

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#6
You can also combine several 1/2 wave dipoles at the feedpoint, one each for each band you are wanting to listen to. Keep the wires inside a PVC pipe, and separated from each other as much as possible. Each resonant dipole will pick up the band its cut for, and the others will be relatively ineffective. I have used this type dipole on HF, where I had 10, 15, 20, and 40 meter dipoles all connected to one coax. It worked quite well. You could experiment with it, and see how it works. Its basically the same theory they use for the ST2 scanner antenna.
Isn't that the design concept behind the Austin Ferret? The scannist community has a love/hate relationship with that antenna...
 

davenlr

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#7
You have to be careful with odd multiples like a 40m dipole will have a low impedance on 15m and if you parallel a 15m dipole with a 40m the RF current will flow on both dipoles and the feedpoint impedance will be about 1/2 of one dipole.

Even multiples are usually fine like a 40m and 20m dipole together where the 20m is essentially out of the circuit on 40m except for some added capacitance between elements and to ground from the extra wire.
prcguy
Yea, you are correct. I used the 40M dipole as the 15M antenna. I had three wires, 40, 20, and 10 if I remember correctly. Its been a while. I had it in the attic of the barracks at Howard AFB in the Canal Zone back in 77.
 

2guntom

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#10
Wow...

okay, so the off-center dipole is out.

From all this information, I can see why a crazy looking antenna like the ST2 works well... fascinating.

Sorry about the balun thing; I didn't know what I was talking about. All I know is that they are in many different antenna designs; I guess I didn't understand their complete function.

?=> Another question I had was about "infinite ground plane". Is this how a 1/4 wave magnetic mount antenna works so well?

Let me inject the history here.

My dwelling has a metal roof. There are also power lines overhead. With this and other metal in the construction, an indoor antenna gives poor results. I am in TN, so terrain is an issue, always. So, antennas have to be outside, but low.

A simple 1/4 wave magnetic mount antenna on the roof is amazing! I had 2 of them (one 1/4 wave, one 1/4 wave w/ a coil [dual band]) and used them for 2 different scanners. I added a third magnetic mount antenna for another scanner; it is 16" and is supposed to be a "multi-band" scanner antenna.

One day I heard radio traffic on the city PD frequency 154.725, something to the tune of somebody "working on" their radios. The signal quality went down immediately. I thought I might move my antenna to improve the signal quality. I discovered that the antenna (1/4 wave dual band) was damaged at the base- UV damage due to cheap plastic. When I moved it, all reception got worse. The other 1/4 wave, I thought best to just leave it alone. The multi-band always had shown a propensity to only like strong VHF signals, and for some reason 800MHz.

After research, based on most bang for the buck, I bought 2 "marine band" antennas. One is a 38" base-loaded, the other a 5' fiberglass whip. They each have coax and mounting brackets so it was easy.

I then disassembled the broken 1/4 wave dual bander. Very poor design, wires just mashed between washers, rust from water getting into places it shouldn't, just pitiful; can't believe I actually spent money on it, but it did work very well in its prime on all the bands I monitor.

I took the coax and soldered an 18" piece of wire to the center and to the shield. The wires were taped to dowel for support, and I put this in 1/2" CPVC. The "wire" I used was from a 6' extension cord.

In my research I found where a guy took a duck antenna, enclosed it in PVC and added coax to make it an outdoor antenna. I just happened to have one of those coax antennas that Uniden used to send with their cheap mobile scanners, the thin coax line that had 24" of the center exposed and suction cups for window mounting.

I cut the suction cups off, taped the exposed center to a dowel, and encased it in 1/2" CPVC.



The 38" Marine antenna is hooked to a PRO 2026 that seems to be extremely sensitive. This is tied to my internet feed. It seems to pull in the 154.725 city PD well and also the 154.37 city Fire. The animal control nazis are on 860.6375; the signal quality is sporadic.

The 5' fiberglass Marine antenna is hooked to 2 scanners. City PD and Fire come in good. There is another station that I am picking up on 154.725; I have to use the tone squelch to block it. The nazis on 860.6375 come in real good too. I am also amazed at some of the distant 800MHz and 460MHz signals that it pulls in.

The 1/4 wave mag mount is hooked to 2 scanners. It picks up about the same as above. This antenna is actually trimmed to 2 meter ham so it picks up that band better than the 5' Marine, but everything else is pretty much the same.

The dipole and the 24" coax antenna- the dipole seems a little better, but in light of all the information in this thread, why does the 24" antenna work for anything? I just don't get it...

The dipole does not pull in city PD well, it is outdone by the others for 2 meter ham, but it seems to do okay in 70 cm ham, 460+ MHz, and fairly well in 860 MHz. Is this because of length, extension cord wire elements, and possibly orientation on the side of the house?

The 24" antenna beats a duck in the house, but was nothing to brag about otherwise. I used it for strong VHF signals (154 MHz) and scanning AM aircraft band. Today I conducted another experiment- 2 scanners, 2 Tee connectors, 1 jumper between the scanners, the 24" antenna on one Tee, and the 16" multi-band on the other. 2 antennas that had no bragging rights, joined together are pulling in as many 2 meter ham repeaters as the 1/4 magnetic, and more than the 5' Marine or dipole.

The only constant I am seeing in "best" results is 1/4 wave magnetic mount.

?=> Is that because of the metal roof? Is that "infinate ground plane"?

?=> If it is, then should I build a 1/4 wave (19") verticle attached to the coax center, then take the coax shield and attach/jumper it to the metal roof?

Or is that a waste of time? Am I missing the point somewhere?

The frequencies that get tough to pull in are
145.47
146.7
146.94
154.37
154.725
860.6375

all others are relatively easy.

I also like monitoring
144-148
150-156
440-450
220-225
460-470
850-865
and trying to see if anything exists above.

There are 8 scanners here, plus a 2 meter mobile that just came from mothballs.

?=> Would it be better to build a dipole specified for each band and dedicate a scanner to it?

?=> Multiple elements in one PVC pipe-style dipole?

Lots of questions, I know...

I actually broke out the ARRL Handbook the other day and tried reading about feedlines and antennas; I think Greek or Hebrew would have been more understandable. I did sleep quite well for about an hour and a half though ;)
 
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#11
A metal roof is a good thing for vertical antennas. Is it flat or peaked? From the list of freqs you want to monitor I would think an NMO mobile mount attached to the roof and a tri band antenna like the Larsen or Maxrad BMXSCAN1000 would do fine. They are both roughly equivalent to a 1/4 wave whip on 150/450/800MHz.

For more $$ the Austin Spectra might do a little better but for actual gain on any band you would probably have to go with separate gain antennas on each band with an appropriate combiner. Once the antenna situation is finalized you could feed all your scanners from one antenna using a multicoupler like a Stridsberg MCA208M, which has an amplifier to make up for splitter loss.
prcguy
 

davenlr

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If you are wanting to BUY an antenna, you cant go wrong with a Hustler 2M collinear antenna for $39. Its a mobile antenna, but mounted over a metal roof, it will give you some real gain on the 2M and VHF high band. Its a standard thread mount, so you would have choices of many mag mounts, trunk lip mounts that might fit on the vent peak strip of your metal roof, etc. They sell a ground plane kit for it as well, if you wanted to get it up higher on a mast.

For the other frequencies, you can either build separate coaxial dipoles and enclose them inside PVC, one for each band, or copy the ST2 design and brew your own. An off center fed dipole (48" on one end, 18 on the other, fed with a 300 ohm to 75 ohm outdoor balun would give you good coverage for VHF and UHF. Most 800 Mhz systems are strong, and you should get them with just about any antenna, unless you are trying to DX a system from out of town.

I just bought ST2's, since they were cheap, came with coax and adapters, and I was in a hurry to get them up. I just started monitoring 1090 MHz aircraft transponders, and the ST2 works pretty darn well even up that high in frequency. Its probably the best affordable all around wideband antenna you can get these days. There are others, the DND Omni, Austin Ferret that I havent tried, but others swear by them. Just a cheap $20 Cell phone mobile mag mount (designed for 700-900 and 1900 Mhz by wilson) works real well on those bands with gain, and VHF as a quarterwave (seems it ignores the coil being the frequency is so low compared to the design frequency).

One point on your 19" quarterwave. Yes, you could do that, but remember that the distance from the whip to the metal roof needs to be relatively short, and the coax center conductor will add itself the the length of the antenna from the point it exits the shield.

Another easy to build antenna, is a simple discone. It has no gain, but covers a lot of territory, from 140 Mhz to 900 Mhz would be an acheivable range if you use a solid disc, or at least 8 elements to form the disc, and the same for the cone.
 

2guntom

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The roof is peaked...

I'm going to stick with home-brew antennas, if at all possible.

Well, good; I'm going to get some more pieces/parts and make some quarter waves and ground the shield side to the house and see how that goes...

And the mystery of the City PD is getting interesting. They sounded good all day, but as the evening waxed on, their signal turned sour. Then, somebody said, "let's all go to channel 2 so we can hear each other", then, the signal quality was perfect. So, it's not just me; they can't hear themselves...
 
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