Homemade On-Center Feed Dipole

RedPenguin

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I currently have this Antenna but modified (the indoor one with balun):


I am currently using it with two 18" legs as that seems to work better for the 453-460MHZ range than the original design in the Wiki.

I also have been using a CATV Amp that seems to really bring in channels really well with little to no noise compared to the antenna by itself.

It feeds a BCT15, BR330T, and two BC75XLTs.

I do have the scanners hooked to a PC via 3.5mm cables.

Anyway some of the further away channels experience a strange behavior.

They will come in extremely clear then for a split second breakup like an audio gap then sound clear again.

I am just not sure what would cause this behavior because bad reception seems to usually be staticy audio but the audio is entirely clear minus the random gaps.
 

RedPenguin

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Also I know placement is key as they say but why is just a few inches even so drastic?

I have the antenna on my curtain rod and it seems on the top right is perfect but even move one of the legs slightly, drastic difference in signal.

I expect moving the antenna by feet but just an inch?
 

majoco

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You have made a dipole - two equal lengths of 18" - the centre impedance is now 72ohms and the resonant frequency just over amateur 2metre band - say around 160MHz, nowhere near the 'broadband' characteristics of an OCFD. You can't drastically change someone's design and then expect it to perform at some other frequency.
 

Ubbe

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That 18" corresponds to a 156Mhz design but it will work almost as good at an odd multiple, like 3 times that are 467MHz. You manage to find the correct lenght where it still will work for you.

Can the audio gaps be heard on all scanners and in their own speakers as well? if you only hear it in the PC then it migh be some type of DC blocking in the PC's audio circuit.

/Ubbe
 

RedPenguin

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That 18" corresponds to a 156Mhz design but it will work almost as good at an odd multiple, like 3 times that are 467MHz. You manage to find the correct lenght where it still will work for you.

Can the audio gaps be heard on all scanners and in their own speakers as well? if you only hear it in the PC then it migh be some type of DC blocking in the PC's audio circuit.

/Ubbe
It was in the scanners themselves but I made some great progress.

I tried after some advice from a chat user on another site, did a roughly 6 inch each set of legs to make about a 1ft dipole.

This was after dipole calculators said this was roughly what 453-460 should need.

Almost every frequency has cleared up amazingly.

A local frequency 453.150 about 11 miles away hasn't but it's only 50W and meant for a campus not far away listening. 453.300 also about 11 miles away sounds mostly clear but a little static that then clears up. That station probably points away from me as it's serving the Northern half of the county and I'm near the bottom of South.

I know I tried the 12" dipole before but it didn't seem to work but I maybe had a bad balun before.

The other thing I noticed was, the BC75XLT with the most channels seems to need an FM Trap on it directly.

I tried putting the Trap on the entire setup (before and after the amp) but that ruins some channels like 453.475 on the BR33.
 

RedPenguin

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Well I made some interesting progress.

The following seems to work:

453.475MHZ about 11 miles away works best with the 1 ft dipole near the top of the window. Near the top of the window is best.

Yet 453.300MHZ and 453.150MHZ (also 11 miles away but different direction) seem to work best with the original 36" dipole originally mentioned in this post. Near the middle of the window is best.
 

RedPenguin

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Just a small update, it turns out the top left of the window with the 36" Dipole works better than middle of window.

I guess will just have to keep two separate dipole up because it seems 453.475MHZ only likes the 12" and 453.300 and 453.100 only like the 18".

For some reason all 3 frequencies act up when I try the actual OFCD in the Wiki.
 

Ubbe

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For some reason all 3 frequencies act up when I try the actual OFCD in the Wiki.
OFCD are a compromise to make a dipole work at two frequencies, either in the same band to make it broadbanded or at different bands. But it will do that to the expense of performance. It can never work as good as a standard single frequency dipole as only half of the OFCD are operational at one frequency, more or less half the efficiancy of a standard dipole.

/Ubbe
 

RedPenguin

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I still have a single issue that I can't seem to completely fix.

Lately, the signal will be esentially crystal clear on virually all of the frequencies then for anywhere from 2min to an hour or so will get bad when the antenna was not touched at all.

453.300 for example can one minute be completely excellent signal then maybe 2 minutes later breaking up then 3 minutes later be perfect again.

453.475 and 453.150 can also do similar, one minute sound excellent then can be randomly static filled then clear up within 5-20 minutes.

The only way I can describe the behavior easily would be like if somebody randomly unplugged the antenna at a times but then plugged it back in shortly after.
 

Ubbe

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It usually happens when a transmitter on another frequency are too strong and overloads the scanner so it will loose sensitivity. But happens only while that transmitter are keyed but if it goes on for an hour it probably must be something else.

Can you poke the antenna and coax with a stick while you listen to check that there's no connection problem.

Every scannerist should always have a FM broadcast filter at hand to test with to see if that helps. It only takes one drop to make a cup overflow and that could be FM broadcast. Also a variable attenuator are also usefull to test if a few dB attenuation helps and then some other type of filter might be considered.

Any amplifier used needs to be compensated with an attenuator at a scanners input to dial in the exact total gain that can be used without getting overload in a scanner. If the amplifier are of low quality it could overload in itself that requires FM trapfilter or other arrangements inserted in front of it.

/Ubbe
 

RedPenguin

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Well, I made some great progress.

I did play
Every scannerist should always have a FM broadcast filter at hand to test with to see if that helps. It only takes one drop to make a cup overflow and that could be FM broadcast. Also a variable attenuator are also usefull to test if a few dB attenuation helps and then some other type of filter might be considered.

Any amplifier used needs to be compensated with an attenuator at a scanners input to dial in the exact total gain that can be used without getting overload in a scanner. If the amplifier are of low quality it could overload in itself that requires FM trapfilter or other arrangements inserted in front of it.

/Ubbe
Thanks again for the info, I was messing around with my RS FM Trap and I thought it worked at first but I checked again location of the one antenna and it seems the 453.300/453.150 really likes hanging on the wall instead next to the window. It's even clearer than before and I think that was the issue.

For some reason the 453.475 dipole refuses to work well unless hanging above the window itself. I still have to be careful as it seems even the slight movement of one of the legs can go from excellent signal to rather staticy.

I think though, both dipoles don't like being near each other as I think that causes a tad of interference to both if they are too close.
 

RedPenguin

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Well I made significant progress thanks to this post (I wanted to thank the user but post too old and says user not seen in years.):


I was having issues where one scanner/sound card kept bleeding in to each other at times.

I tried everything from replacing audio cables to coxial cables but the problem only went away for a short time or just was less problematic.

Finally running a wire from my PC case on one end then to one of the scanner's BNC adapter, seems to have finally ceased all of the bleedover for hours now.

I originally tried putting the wire to a nearby metal service with no luck because the PC is across the room from the scanners but it had to be the PC Chassis itself.

So far 453.300, 453.150, and 453.475 all sound incredibly better and I haven't seen gaps in audio.
 

RedPenguin

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Ok now I have a strange issue.

It seems like Bedford County 453.475 is interfering with my receiving of my local county's 453.475.

I noticed before it got severe, you could actually hear the interference in Cambria County's version. I actually heard users talk, then a garbled communication would come through, and the users would be like "What was that?".

Today 9am, I had perfect signal but now I keep getting what sounds like PD Dispatches from Bedford County.

Even removing the amp isn't working.

I think something is going on because even on 453.300 strange voices are happening and the a County Dispatch & an officer said "Who was that voice?". I don't know which 453.300 would bleed as Bedford doesn't use that frequency.
 

RedPenguin

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Well, something obviously changed.

I noticed that around 11PM, my county did some "testing" checks where the dispatchers did audio checks.

Since then 453.300 is the best reception it's ever been along with the other channels coming in nice and clear like 453.150.

I completely took off the amp for 453.475 and so far so good.
 
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