More antenna questions

[cabfeegig]

I wanted to get folk's opinions on a couple of questions: (for recieve only):

1 - What is the best wide band scanner antenna for atic or inside the house - OCD (66" or other length) o, multiple dipoles cut for specifc bands, or a J-Pole? Listening to 120->900 mhz.

2 - Can I attach multiple home grown antennas via 300 ohm (like an OCD and then a dipole cut for MILAIR) together to a balun and then to RG6, or will they muck each other up, or should they be separate runs then brought together via a reversed signal spliter?

3 - Where doe the 66" length come for the OCD sold by Grove and the home made one? I know the OCD is a 1/2 Wave length driven approx .33 percent off center. Is the frequency used to caclulate the base of the target frequence? So for Milair, it would be a total of 18.72" (assumeing 300mhz) driven at 6.23" from one end?

I used to be into scanners 20 years ago and have gotten the bug again. I have a BCD 396T with the RS 800 mhz that I used maily for trunking in the Tampabay area. I used an old PRO-26 for everything else.

thanks!

 

[jonny290]

Thing about receive setups is that SWR/impedance doesn't matter *too* much. Now, if you have an impedance of 2 ohms and reactance of 2000 ohms, your scanner won't be happy and won't pull in much. But generally, any VHF antenna is going to get you 'close enough'.

I'm not sold on OCFD's yet, or at least the 66" ones. I still maintain that their natural resonance (86-88 mhz) allows FM intermod to come through too strongly. I'd make an OCFD centered around 120 mhz for milair, may be better.

J-poles are also good, I'm a big fan of them. Need a lot of tuning, though.

You won't really be able to just combine antennas by hooking them through a splitter - because they occupy different physical spaces and have different cable lengths, the signals from each will be out of phase by a certain amount at certain frequencies and you'll have no (or very deep nulls in) reception.

 

[N_Jay]

What is the assumed benefit of an OCFD?

 

[jonny290]

300 ohms feed impedance and 'good performance' on several frequencies. basically a dipole fed at the 300 ohm impedance point, when cut for HF you can resonate them on several bands. i'm not really convinced that they're better than just a plain old vertical dipole cut for your lowest frequency, there's balun losses involved in an OCFD and unless it has a db or two of gain over a dipole, you're at a net loss

i'll do an OCFD impedance chart later on this evening to show you guys how far off the impedance is, though. it's pretty gross

 

[N_Jay]

300 ohms feed impedance and 'good performance' on several frequencies. basically a dipole fed at the 300 ohm impedance point, when cut for HF you can resonate them on several bands. i'm not really convinced that they're better than just a plain old vertical dipole cut for your lowest frequency, there's balun losses involved in an OCFD and unless it has a db or two of gain over a dipole, you're at a net loss

i'll do an OCFD impedance chart later on this evening to show you guys how far off the impedance is, though. it's pretty gross

That is what I thought.

Some people seen to attach "magic" properties to antenna designs.

 

[cabfeegig]

Thanks for the replies... Do you think an OCD would be better or worse (I know broader) than a dipole for MILAir? Is it possible to tune a J-pole for scanner stuff manually (maybe a dumb question!)

 

[N_Jay]

Thanks for the replies... Do you think an OCD would be better or worse (I know broader) than a dipole for MILAir? Is it possible to tune a J-pole for scanner stuff manually (maybe a dumb question!)

I don't see any reason that a OCFD would be any broader then a CFD.

The gain on a dipole is only slightly better (probably not noticeable to a scanner) than a ground plane.

A properly constructed discone is only slightly worse, (probably not noticeable to a scanner) with MUCH wider bandwidth.

 

[kb2vxa]

Hi Cab and all,

I think you can see where this is going but I'd like to have a little fun with it.

First, a J-pole is band specific and so is a dipole no matter where along it's length it's fed so you'd need a bunch of them. You could build a fan dipole that would do the trick and end up with a mutant Scantenna that has a frequency vs. gain curve that looks like mmmmm but why confuse yourself? Then you could Frankenstein a bunch of antennas together and have the unpredictable results drive you crazy. In any case you'd eventually give up and shoot yourself which would really piss off your wife, she couldn't collect your insurance because of the indemnity clause. She'd have you cremated while screaming "Burn in hell you bastard!" which wouldn't sit too well with the family.

Now that I've had my fun you're probably thinking about going the proscribed route with a discone which pretty well suits your intended purpose. It's wide band, compact design fills the bill, never mind gain because you'll never fit a collinear or a beam in the attic. Of course never scrimp on the coax but you knew that, didn't you?

Now N_Jay, since the dipole, ground plane and discone are all variations on a theme, what makes you think they have different gain characteristics or any gain at all? The word the tech manuals use is "unity", something you may have missed in your studies. It just could be that's why it's "probably not noticeable to a scanner" don't you think?

 

[N_Jay]

Now N_Jay, since the dipole, ground plane and discone are all variations on a theme, what makes you think they have different gain characteristics or any gain at all? The word the tech manuals use is "unity", something you may have missed in your studies. It just could be that's why it's "probably not noticeable to a scanner" don't you think?

"Gain" is a relative term, and if you notice I only used it to show the relationship between antennas.

Unity is a misnomer, unless you provide the reference.

Maybe you need to study a bit more. They all have 20 to 100dB GAIN over a dummy load, and about 6 to 15 dB loss when compared to a small band specific yagi.

"Tech Manuals" are good, but they often simplify complex concepts. Sometimes you have to learn from "Text Books" to get the whole picture.

 

[prcguy]

Another problem with using an OCF dipole for wide band scanner use is the radiation pattern will have serious nulls at the horizon at the upper frequencies due to the elements being too long. Even the short 18” element shown in the Wiki is 1.5 wavelengths long at 850MHz, which will have a “hole” that can easily be 10dB or more down at the horizon from what you would get from a 1/4 wave ground plane. I wouldn’t be surprised if the wires between the matching transformer and antenna are the dominant source of its 800MHz performance. The OCFD may have gain here and there but that gain could be upwards or downwards at a 45 or 60deg angle where it has no benefit. The classic use for an OCF dipole is in the HF ham bands where the antenna is horizontal and the various gain lobes can be put to good use. The only truly wide band antenna that is affordable for scanner use is the Discone (here comes the death threats). When I say wide band, I mean it’s a real antenna continuous from VHF air to 800MHz and everything in between. Except for some exotic military stuff, nothing else on the ham/scanner market will provide as much useable bandwidth in an omni directional vertical pattern. The Discone does have problems at the upper freqs where the pattern tilts up, so its not the best choice for weak 800MHz stuff, but at lower freqs its about as good as a resonant 1/4 wave ground plane.
prcguy

 

[N_Jay]

. . The only truly wide band antenna that is affordable for scanner use is the Discone (here comes the death threats). When I say wide band, I mean it’s a real antenna continuous from VHF air to 800MHz and everything in between. Except for some exotic military stuff, nothing else on the ham/scanner market will provide as much useable bandwidth in an omni directional vertical pattern. The Discone does have problems at the upper freqs where the pattern tilts up, so its not the best choice for weak 800MHz stuff, but at lower freqs its about as good as a resonant 1/4 wave ground plane.
prcguy

Of course you mean a well designed discone used within its true 3:1 (maybe 4:1) bandwidth, and not the 25 MHz to 1300 MHz advertised bandwidth.

 

[prcguy]

Yes, I meant a well designed Discone, which has a legitimate 8:1 frequency range although the radiation pattern gets a little squirrelly at the top end. The RS, Diamond DJ-130 and a few other Discones have a low end cut off around 100MHz and lower, which would put the top end around 800MHz at best. This is why most people complain of the Discones poor 800MHz performance, there is probably a big hole at the horizon and any useful lobes are pointing up too high. Also, most scanner Discones don’t have enough elements! The design criteria for Discones made with rod type elements is the spacing of the rods at the bottom of the cone must be no more than .02 wavelengts apart at the tips at the low cutoff frequency. This is to simulate a solid disk and solid cone and requires at least 16 elements (if my memory is correct) and most have only 8, which is a compromise in performance. Commercial Discones from Kreco, Astron and the military AT-197 have 12 elements and are closer to the original design.
prcguy

Of course you mean a well designed discone used within its true 3:1 (maybe 4:1) bandwidth, and not the 25 MHz to 1300 MHz advertised bandwidth.