Homebrew 800mhz(trunking) antenna HELP!

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Shortwavewave

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Ok, I have a (new to me) BCT8 scanner, It came with a friggin small 4in rubber duck antenna.

I wanted to build a homebrew 800mhz indoor or outdoor antenna....Heres my problem anything Ive built doesnt outpreform this small rubber one.

SOMEONE please tell me what make it better? its not a full wave lenght
(what makes, or how do you know if a smaller wave is going to work better than a full wave on a perticular freq?, to me it makes more sence to have a full wave because thats how long that actual radio wave is, right?)

correct me if im wrong, 850mhz is approx for my local systems
984 / 850mhz=1.15 feet Full wave
492 / 850mhz=5.2inches Half Wave
246 / 850mhz=2.9inches 1/4 Wave

I have an outdoor antenna(8ft dipole) that worked great for me on EVERYTHING(literly i mean everything), untill I finally got a trunking scanner, now Im confused, because when I hook up that outdoor antenna I cant even hear the control channel,
noise overrides it.

One of the antennas I tryed to make was a 10inch dipole on each side with a 3sheild coax(has foil, braid, and another foil), and I still couldnt hear the control channel.

Can some one reccomend a homebrew antenna, or what I might be doing wrong, and clear stuff up for me?
THANK YOU IN ADVANCE

EDIT: forgot to mention YES the rubber duck preforms well but everything is still staticy, and distorted, i need somthing to improve that.
 
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fineshot1

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A full wave & half wave antenna requires an impedance matching
device as they are not gonna be 50 ohms if you simply cut them to
the full & half wave lengths and try & use them like that. A 1/4 wave
will be at about a 50 ohm impedance and you can make a ground
plane 1/4 wave antenna real easy. Just use a N connector chasis
mount(has the screw mount holes in the corners) and solder a
about 3 inch wire into the center conductor and also a slightly
longer wire into each of the 4 corner screw holes and angle
them down ward at about 45 degrees and you have a 1/4 wave
800mhz ground plane. If you make a little currley half circle at
the top of the virtical element you can hang it from a string
much like a planter can hang from one of those web hangers.
EDIT: You can get some great antenna building info from the
ARRL Antenna Handbook sold online on many web sites - just
do a google search and you will get many many hits.
 
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Shortwavewave

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The best homebrew Ive come up with is this (it works better than the rubber duck, but not as good as I want, and I know im not going to be able to be perfect)

I took a Wilson1000 CB antenna and cut about 8inches off the top and put that on the bottem(used a quad shield) resoldered the coax to the bottem of the antenna, (which theres a coil in the plastic but it wont come apart) and some how the shield is connected to center conducter somewhere throughout the coil because, I took my multi meter and tryed to test the ohms, and it wasnt reading 50(or because my meter is off it said 10.2) like it should, but was showing nothing with the antenna connected, so I checked the(beeper, conducter tester what ever you want to called it)
and the ground of the scanner, and the antenna whip(center conducter) are connected??? WTH?

SO im asumeing the wilson1000 has some sort of coiled loop? and with that 8in whip I am picking up better than the rubber duck, so Ive gone in the right direction, just dont know how far to go.................lol
 

fineshot1

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NJ USA (Republic of NJ)
Most likely that antenna is a closed coil model so any attempts at checking for a short is not
going to yeild much in the way of results. Best to test closed coil antennas with a swr meter
or antenna analyzer.

You can not measure impedance with a multimeter in any ohms range selection.

Impedance is a reaction in a circuit at the frequency range the antenna was designed for
or even when its being used out of its designed frequency range. That is why I recommended
the ARRL Antenna Handbook as it has all kinds of explanations of antenna design & theory so
you can read up and learn about that sort of information.
 
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ka3jjz

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Bowie, Md.
Anything in our scanner antennas wiki, including the AC6V supersite, that includes the formula for calculating lengths and spacing should, at least in theory, be usable to design a 800 mhz antenna. But for sheer simplicity, as fineshot mentioned, a ground plane is hard to beat. At these freqs, even a small Yagi is so tiny it's almost ridiculous.

73 Mike
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
Others have mentioned matching problems with anything other than a 1/4 wave ground plane or dipole. The dipole can be the classic 1/4 wave elements perpendicular to the coax or a sleeve dipole where you expose 1/4 wavelength of center conductor and peel the braid back over the coax for 1/4 wavelength. Besides the matching problems with 1/2 wave end fed whips, full wavelength whips and other large wavelength antennas is the radiation pattern. A ground plane or vertical 1/2 wave dipole will transmit and receive much of its energy out sideways towards the horizon where you want it to go. When you get over 1/2 wavelength the pattern changes and every time you add another 1/2 wavelength the pattern splits and another lobe appears somewhere other than at the horizon. A full wave whip will have its maximum null (dead spot) at the horizon and all the energy will go up and down at 45deg angles. Not so good for most communications. To add gain, multiple elements can be combined and stacked at the proper distance apart so the elements operate in phase and their contribution is beneficial rather than destructive. Antennas are very complex and unless you follow certain design rules, you can spend a lot of money and energy making something and end up with a lot of metal in the sky that resembles an antenna but is much better suited as a bird perch.
prcguy
Ok, I have a (new to me) BCT8 scanner, It came with a friggin small 4in rubber duck antenna.

I wanted to build a homebrew 800mhz indoor or outdoor antenna....Heres my problem anything Ive built doesnt outpreform this small rubber one.

SOMEONE please tell me what make it better? its not a full wave lenght
(what makes, or how do you know if a smaller wave is going to work better than a full wave on a perticular freq?, to me it makes more sence to have a full wave because thats how long that actual radio wave is, right?)

correct me if im wrong, 850mhz is approx for my local systems
984 / 850mhz=1.15 feet Full wave
492 / 850mhz=5.2inches Half Wave
246 / 850mhz=2.9inches 1/4 Wave

I have an outdoor antenna(8ft dipole) that worked great for me on EVERYTHING(literly i mean everything), untill I finally got a trunking scanner, now Im confused, because when I hook up that outdoor antenna I cant even hear the control channel,
noise overrides it.

One of the antennas I tryed to make was a 10inch dipole on each side with a 3sheild coax(has foil, braid, and another foil), and I still couldnt hear the control channel.

Can some one reccomend a homebrew antenna, or what I might be doing wrong, and clear stuff up for me?
THANK YOU IN ADVANCE

EDIT: forgot to mention YES the rubber duck preforms well but everything is still staticy, and distorted, i need somthing to improve that.
 

Shortwavewave

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Joined
Feb 20, 2007
Messages
514
Thanks for clearing that up for me prcguy!
(I dont spend alot of money on antennas, unless I know there something that works and I really want)

Ive looked over the net for "building 800mhz antennas" and not a hole lot from ham radio sites.

If I wanted to build a 800mhz yagi, how would I know to use a dipole, or a loop coming off the coax(drivin elment i think its called)?

Im thinking of useing KF4LNE's http://www.radioreference.com/forums/showthread.php?t=45999 (thanks BTW) $5 yagi from coat hangers and dowl and just cutting it to length
 
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k8tmk

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Jan 13, 2004
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Location
Stevensville, MI
To build a cheap 800 MHz yagi, look at an article in Popular Communications (about a year ago issue). It had several different configurations of yagis (number of elements). I built the biggest one (only about 3 feet long) and it works great. I got everything I needed (except the coaxial cable for the lead-in) from Ace Hardware.

I can't tell you exactly which issue of the magazine the article was in because I already packed up my 2007 magazine issues and stored them away.

Randy, K8TMK
 
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