Homebrew Off-Center Fed Dipole Horizontal vs Vertical?

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RedPenguin

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A while back, I made this Antenna:

Homebrewed Off-Center Fed Dipole - The RadioReference Wiki

I made the "portable" version using the bell wire.

It seems to state that the antenna is supposed to be vertical but oddly when I want to listen to 453.475 and 460.375, it seems to really only work horizontal.

I was wondering why this would be or does it have to do with the band?
 

prcguy

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Probably because its radiation pattern is lousy at the horizon on UHF with lobes pointing up and down and if you turn it on its side you can point a gain lobe at what you want to hear. Try turning it in horizontal mode and see if you get a peak and a null on the signal.
prcguy

A while back, I made this Antenna:

Homebrewed Off-Center Fed Dipole - The RadioReference Wiki

I made the "portable" version using the bell wire.

It seems to state that the antenna is supposed to be vertical but oddly when I want to listen to 453.475 and 460.375, it seems to really only work horizontal.

I was wondering why this would be or does it have to do with the band?
 

Ubbe

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It could be that you have a strong transmitter that desense the scanner and going horisontal will attenuate the signal at least 10dB and directing the antenna to minimize the signal will attenuate it even further to the point where the front-end of your scanner begins to function again.

Have tried one of your other scanners?

/Ubbe
 

RedPenguin

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I have not had a chance to attempt it on another scanner, but I did notice something interesting.

Leaving this antenna in horizontal mode and adding an FM Trap helped a fair amount, even though I never noticed any FM interference before, but I do have 8 FM stations that are about 2.5 miles away.

Also just to see what would happen, I tried two of my cable amps I had lying around.

The first was a 10dB one with no return path and had a 4 port splitter in it.

I noticed very little difference but oddly when I put my 25dB one that has only one output with Return Path, all the sudden, the channels I have had rather bad reception even though they are only 19 miles or so away, came in crystal clear like the much closer 2 miles or so stations.

Neither amp seemed to work well without the FM Trap before when I tried using them a while back.
 
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popnokick

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Could be an intermittent connection or short. Have you checked all of the coax, connectors, any jumpers, and other connections with a multimeter.... wiggling them as you observe the short / open indication?
 

dlwtrunked

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I have not had a chance to attempt it on another scanner, but I did notice something interesting.

Leaving this antenna in horizontal mode and adding an FM Trap helped a fair amount, even though I never noticed any FM interference before, but I do have 8 FM stations that are about 2.5 miles away.

Also just to see what would happen, I tried two of my cable amps I had lying around.

The first was a 10dB one with no return path and had a 4 port splitter in it.

I noticed very little difference but oddly when I put my 25dB one that has only one output with Return Path, all the sudden, the channels I have had rather bad reception even though they are only 19 miles or so away, came in crystal clear like the much closer 2 miles or so stations.

Neither amp seemed to work well without the FM Trap before when I tried using them a while back.
Remember with pre-amps, it is also the NF (noise figure) that matters. Adding only a pre-amp with high gain AND lower NF is likely to help. In addition, with nearby transmitters (2.5 miles counts), a high 3rd intercept point is often needed. And adding an FM broadcast band trap should always be done even if one thinks one does not have a problem (I have preached that for years). De-sensing by FM broadcast stations is devious and often not realized.
 

RedPenguin

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Could be an intermittent connection or short. Have you checked all of the coax, connectors, any jumpers, and other connections with a multimeter.... wiggling them as you observe the short / open indication?
Even when I rebuilt the antenna when a balun once went bad and even when I built another, it still always prefers to be horizontal.
 

Ubbe

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That 4 port amp could be a distribution amp intended for already amplified high level signals.
My experiance is that 9 times of 10 any antenna amplifier will make your antenna great again.

/Ubbe
 

RedPenguin

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I had another scanner acting somewhat staticy, where it was listening to a close-by frequency but at times the signal would completely drop in to static but come back again.

So luckily adding a 2 splitter to this setup with the GE 25db Amp with Return Path, FM Trap, and Bell Wire OCFD has greatly helped reception even on stuff that is only 5 miles away.

Since the radios are hooked up to my sound cards, I can "examine" the audio in Audacity.

I noticed immediately after adding the amp, that the background noise immediately smoothed out with the most part.
 

RedPenguin

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On-Center Feed Dipole

It turns out the typical RR OFCD wasn't what I needed in my case.

Just as a test after using a leg calculator and finding making an "on-center" feed dipole, that each leg is roughly 5 inches, I tried that.

Oddly the new one likes to be hung vertically vs horizontally, but the reception is insanely better than even the OCFD was getting it.
 

RedPenguin

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Slight update on my last.

For some reason making a "V" out of it seems to work the best.

It's hanging by the window and making a V-shape with the balun in the middle.

I guess I get the best of both worlds, as the V-shape is somewhat horizontal and somewhat vertical at the same time.
 

RedPenguin

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I guess there are really a lot factors to my reception.

I found out hanging the off-center feed dipole back in it's L shape with short leg going like a "V" and the long leg going horizontal like the bottom of a capital L, with the 25dB cable amp with FM Trap, and using cables that are dual-shielded seems to really get reception in clear.

I tried a single cable that was supposedly quad or tri-shielded and that worked worse as did my RG-6's provided by the local cable co.
 

Ubbe

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I suggest you get a variable 0-20dB attenuator to be able to get the sweet spot where the scanners work best and you probably don't have to bother with strange setups of your antenna.

/Ubbe
 

RedPenguin

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Nice....

I suggest you get a variable 0-20dB attenuator to be able to get the sweet spot where the scanners work best and you probably don't have to bother with strange setups of your antenna.

/Ubbe
Thank you for your suggestion.

I haven't realized variable attenuators were around but I had heard of the inline ones that are set to a particular dB such as 6db.

I think you hit the nail on the head as they say because even using a cheap Dollar Tree 2-splitter or higher quality 2-splitter really evens out the signal a lot, which is obviously attenuating the signal to a degree.

I ordered one already, but I am surprised how difficult they are to find compared to the typical inline ones.

Even a YouTuber said variable attenuators should be in any TV antenna person's toolbox.
 

RedPenguin

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I am still waiting for my variable attenuator to arrive.

I did end up fixing the antenna by finding that RFI was being caused by another scanner of all things.

I had an older Regency Radio Scanner plugged in to another room in a power strip with a cheap alarm clock, floor fan (was turned off), and a touch lamp plugged directly in to other outlet (also turned off).

For some reason that combination caused this noise on 453.475:

https://www.dropbox.com/s/cxkm6m1nojne0j0/click.mp3?dl=0

But plugging the Regency directly in to the outlet vs the strip still caused noise, but mysteriously not physically moving the Regency but plugging it in to another outlet nearby, completely stopped the RFI.

So now the OCFD is picking up a fair amount better also, being the fact I replaced the 24 AWG wire with 12 AWG wire also and used a much better expensive balun.
 

popnokick

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Glad things have improved and you've found the source of your RFI (hopefully all of it). That's one thing that frequently gets overlooked with regard to wide frequency coverage antennas like discones and OCFDs: Yes, they are very good for receiving a very wide range of different frequencies, but also good at receiving RFI and bringing it straight into your receiver... no matter that the RFI is well out of the band where the receiver is tuned. And of course many people are limited in antenna locations and and cannot put an antenna outside.... so the wide coverage antenna is placed INdoors... much closer to the usual sources of RFI garbage. So anyone using an indoor antenna should be asking, "Hmmm.... wonder if I'm being affected by RFI from something here inside the structure?"
 

RedPenguin

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Yes

The strange part in my case was, even if you took a handheld around with just a small rubber duck antenna, it got the interference from the Regency radio all around the place.

It was picked up on various radio scanners no matter what antenna they had.

After talking to somebody who knows a bit about RFI, they think somehow the outlet the Regency was plugged in to must have somehow let the power wires act as an antenna as they said they wouldn't expect the interference around the whole place no matter what radio and antenna.

EDIT: So far the OCFD is doing great now. The main freq (14 miles away) I want to hear sounds almost as good as it's sister channel that's only about 5 miles away. Strange thing is even though I am only using my OCFD indoors, I seem to be getting better reception than some of actual radio users when they attempt to transmit/receive only blocks from my place at times.

Also not sure if it's tropical skip or I just opened bands up by eliminating the RFI, but another county's channel appears to be coming in somewhat now, horribly, but I never got another county's traffic before from my location. It's too lousy of signal to verify it's source but only two nearby counties use the freq.
 
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RedPenguin

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An update to this.

I have found exactly what is occurring with the Regency.

For some reason, any time you plug in 464.175 in the Regency, it causes either various clicks or open squelch like sounds on 453.475.

Sometimes moving the Regency around or messing with the antenna stops it for a short time but then it starts up again.

The family member doesn't want to get rid of the Regency, so is there any way to stop the interference on 453.475 while still allowing it to listen on 464.175?

I thought of maybe a ferrite on the power cord or something, but nothing seems to work.
 

prcguy

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Many scanners use a 10.7MHz IF, meaning they have a local oscillator (internal transmitter) that is tuned 10.7Mhz away from the desired frequency to be received and the mixer in the scanner converts the desired frequency to 10.7MHz for further filtering and processing downstream. Its common for many scanners to radiate its internal local oscillator out through the antenna jack and into the air for a short distance.

Let's see, your 464.175 frequency in the Regency, minus 10.7MHz for its internal local oscillator equals 453.475MHz, what a coincidence!

Try disconnecting any external antenna on the Regency and see if the interference goes away. If so you can probably use a very low gain, low noise amplifier in front of the Regency and that will reduce the radiated local oscillator interference quite a bit. There is a good chance its just leaking through a plastic case on the Regency and aside from shielding the case you might not be able to improve things much.
prcguy


An update to this.

I have found exactly what is occurring with the Regency.

For some reason, any time you plug in 464.175 in the Regency, it causes either various clicks or open squelch like sounds on 453.475.

Sometimes moving the Regency around or messing with the antenna stops it for a short time but then it starts up again.

The family member doesn't want to get rid of the Regency, so is there any way to stop the interference on 453.475 while still allowing it to listen on 464.175?

I thought of maybe a ferrite on the power cord or something, but nothing seems to work.
 
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