Attic Antenna - Yes, Another Thread...

[emsflyer84]

Hey guys, recently set up a base scanner for a friend and he can't have anything mounted outside or on the roof. I gave him a Centerfire folded dipole antenna that I set up in his attic. This is the antenna:

Folded Dipole Scanner Antennas

Now you can get the performance of a folded dipole without the expensive price tag. Our folded dipole scanner antennas provide a wider bandwidth than our standard dipole scanner antennas. They are …

centerfireantenna.com

It's tuned specifically for public safety bands in high VHF which is 90% of what he's listening to. There are a few things he gets in the 400's and they still come in, but probably not as well as they could. There is also a couple distant frequencies that come in weak (analog) and a digital frequency he'd like to get that comes in but it's broken and sounds very "digital" most of the time and it's hard to make out. Just slightly better reception would clear these up.

He's been looking at the OmniX antenna to see if it would improve reception. It's probably an overall better antenna then the dipole, but my worry is that it's wide-band so the stuff in the VHF-high range might actually come in worse then it does now with the tuned dipole. The stuff in the 400's might improve a bit, but the potential loss in VHF wouldn't be worth it.

Any advice here would be great. I use a dual-band diamond commercial grade vertical on my roof at my house and don't have much experience with trying to find an ideal dual band (VHF / UHF) antenna that would work well in an attic. But ideally that's what he'd want.

Thanks guys!

 

[mmckenna]

Usually mounting an antenna in the attic comes with those drawbacks. Most of the frequencies that the scanner covers works by line of sight. Having the antenna down low isn't helping. Having it co-located with the RF noisemakers in the house doesn't help. Having it in the attic with foil backed moisture barrier insulation, HVAC ducting, plumbing, wiring, etc. doesn't make it any better.
But yeah, limits are limits, and we understand.

I tried a Centerfire UHF antenna about 20 years ago on UHF. Was not impressed with it. It worked, but it took a lot of work to get it adjusted correctly.

Those that use the Omni-X seem to love it. I'm not a fan of 'gimmick' antennas. And I think there stuff is overpriced.

If I was setting someone up with a scanner and antenna in the attic, I'd go with a mobile multiband antenna on a simple mount.
I really like the Larsen NMO-150-450-800 https://www.theantennafarm.com/cata...oduct_info&cPath=191_192_205&products_id=1050
or the Larsen NMO-150-450-758 https://www.theantennafarm.com/cata...oduct_info&cPath=191_192_205&products_id=8932

I'd get a piece of sheet metal about 36" across and put a 3/4" hole in the middle with an NMO mount. That would be a cheap/easy ground plane. Or, buy one of the NMO base station adapter kits.

And make sure you have suitable coaxial cable for the length of the run.

If VHF and UHF is all they need, and there's no interest in 700, 800 and 900MHz, then you can get a commercial dual band mobile antenna:

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=191_192_200_451_220&products_id=5583

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=191_192_200_451_219&products_id=862

 

[emsflyer84]

Usually mounting an antenna in the attic comes with those drawbacks. Most of the frequencies that the scanner covers works by line of sight. Having the antenna down low isn't helping. Having it co-located with the RF noisemakers in the house doesn't help. Having it in the attic with foil backed moisture barrier insulation, HVAC ducting, plumbing, wiring, etc. doesn't make it any better.
But yeah, limits are limits, and we understand.

I tried a Centerfire UHF antenna about 20 years ago on UHF. Was not impressed with it. It worked, but it took a lot of work to get it adjusted correctly.

Those that use the Omni-X seem to love it. I'm not a fan of 'gimmick' antennas. And I think there stuff is overpriced.

If I was setting someone up with a scanner and antenna in the attic, I'd go with a mobile multiband antenna on a simple mount.
I really like the Larsen NMO-150-450-800 https://www.theantennafarm.com/cata...oduct_info&cPath=191_192_205&products_id=1050
or the Larsen NMO-150-450-758 https://www.theantennafarm.com/cata...oduct_info&cPath=191_192_205&products_id=8932

I'd get a piece of sheet metal about 36" across and put a 3/4" hole in the middle with an NMO mount. That would be a cheap/easy ground plane. Or, buy one of the NMO base station adapter kits.

And make sure you have suitable coaxial cable for the length of the run.

If VHF and UHF is all they need, and there's no interest in 700, 800 and 900MHz, then you can get a commercial dual band mobile antenna:

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=191_192_200_451_220&products_id=5583

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=191_192_200_451_219&products_id=862

I have used the nice Laird ground plane kit with one of their dual band mobile antennas before with good results at my house so maybe I’ll try that. I just don’t know if it will be a worthwhile improvement over the current folded dipole. I guess all I can do is try... thanks.

 

[12dbsinad]

Before throwing any money at it, hook the scanner up to a battery and kill the main breaker if you can,or shut down lighting, HVAC, etc the best you can. I'm not sure if this is a single family or duplex, apartment, etc. See if reception improves. As mmckenna said, RFI can be pretty bad in a attic, especially on VHF. LED lighting is a huge one. I have new LED puck lights that completely whip out VHF anywhere near them.

If no change, then I'd look into something like a 3db VHF mobile antenna with a ground plan kit or large cookie sheet or piece of metal. A economy 3 db VHF base station antenna would also be ideal if you have the vertical room in the attic as they are rather long.

Just food for thought. Good luck.

 

[CanesFan95]

mmckenna's idea about the sheet metal is something I did and it helped with an attic antenna. I found a sheet metal place in town and ordered a few 2' x 2' pieces and had them cut the 3/4" hole in the middle for NMO antennas.

 

[emsflyer84]

Thanks guys, just for fun I took my Diamond commercial dual band vertical and it it up in the attic to see the difference compared with the folded dipole. Some distant digital vhf high frequencies definitely come in better and more consistently. Some distant analog vhf high frequencies seem to be coming in weaker. Very strange. I need to give it more time to see if something else like atmospheric conditions might just be affecting these analog frequencies right now. We’ll see.

 

[emsflyer84]

Ok guys, very strange situation, but maybe it's normal.... So I've been going back and forth between the folded dipole antenna and a commercial dual-band diamond vertical, both mounted next to each other in the attic. Here's what I've noticed: Distant VHF high analog frequencies come in better with less static with the folded dipole. Distant VHF high digital frequencies come in better and mote consistently with the Diamond. Scanner is the same and antennas are next to each other. Is it possible that one antenna handles digital or analog frequencies better then the other, even in the same frequency range? I just thought it was strange. I'm also surprised how well the $40 folded dipole does with analog VHF. Much less static on distant frequencies, and cleaner reception then the $100+ Diamond dual-bander. But, like I said, the Diamond is pulling in distant digital VHF better then the dipole. Just my observations. The joys of experimentation. Thanks guys.

 

[JimD56]

I run 2 attic antennas 1) Diamond VHF/UHF Discone 2) 700-800mhz Yagi both into a comet duplexer with a 20-foot drop RG6 into an Electroline +15db preamplifier then into an Electroline 8 way +4db preamp into the rack. No issues, great results.

 

[emsflyer84]

I run 2 attic antennas 1) Diamond VHF/UHF Discone 2) 700-800mhz Yagi both into a comet duplexer with a 20-foot drop RG6 into an Electroline +15db preamplifier then into an Electroline 8 way +4db preamp into the rack. No issues, great results.

I'm thinking about running both antennas, but I'm only using one scanner, so I think I would need a DIPLEXER not a DUPLEXER. I'm not sure how that will work out running two antennas to one scanner....

 

[jonwienke]

They are the same thing. Bidirectional devices that segregate signal by frequency on one side, and combine the signals on the other.

 

[emsflyer84]

They are the same thing. Bidirectional devices that segregate signal by frequency on one side, and combine the signals on the other.

So I would just need a duplexer to run separate VHF and UHF antennas into one scanner? Both antennas could potentially be receiving the same signals at the same time. This won't create distortion issues?

 

[mmckenna]

Is it possible that one antenna handles digital or analog frequencies better then the other, even in the same frequency range?

No. There is zero difference in how an antenna performs when comparing analog to digital.

Here's what is likely happening:
The folded dipole would be considered a "DC Grounded" antenna. That is that the center pin of the coaxial connector will show DC continuity to the outer shield. This is normal and part of the design.
That DC ground helps drain off static and can lower the noise.

The vertical may not be a DC grounded antenna (some verticals are, depending on the design), so it may suffer from more noise. But it's going to have higher gain compared to the folded dipole, so distant stations will come in a bit stronger.

Has nothing to do with analog or digital.

 

[krokus]

Is it possible that one antenna handles digital or analog frequencies better then the other, even in the same frequency range?

No. The type of signal is irrelevant, only the frequency. There could be a minor difference in the peak signal frequency of the antennas, which could show as a stronger signal, with all else being equal.

Edit: mmckenna beat me to it.

 

[emsflyer84]

No. There is zero difference in how an antenna performs when comparing analog to digital.

Here's what is likely happening:
The folded dipole would be considered a "DC Grounded" antenna. That is that the center pin of the coaxial connector will show DC continuity to the outer shield. This is normal and part of the design.
That DC ground helps drain off static and can lower the noise.

The vertical may not be a DC grounded antenna (some verticals are, depending on the design), so it may suffer from more noise. But it's going to have higher gain compared to the folded dipole, so distant stations will come in a bit stronger.

Has nothing to do with analog or digital.

Thank you for the explanation. Makes sense. I don't know if the vertical is DC grounded but it does have gain. It's just very strange. The dipole sounds significantly better on distant analog frequencies.

 

[emsflyer84]

No. There is zero difference in how an antenna performs when comparing analog to digital.

Here's what is likely happening:
The folded dipole would be considered a "DC Grounded" antenna. That is that the center pin of the coaxial connector will show DC continuity to the outer shield. This is normal and part of the design.
That DC ground helps drain off static and can lower the noise.

The vertical may not be a DC grounded antenna (some verticals are, depending on the design), so it may suffer from more noise. But it's going to have higher gain compared to the folded dipole, so distant stations will come in a bit stronger.

Has nothing to do with analog or digital.

Well, it turns out the vertical IS DC grounded....

 

[Ubbe]

Well, it turns out the vertical IS DC grounded....

It's a matching net of coils and capacitors that sits in the base of the antenna to give low SWR in the amateur bands and one end of that coil goes to ground.

I modified my dualband diamond antenna by moving the output on the coil to the upper part where the antenna signal feeds the coil to get the maximum signal and lifted one leg of capacitors and bridge others to direct couple the signal to the coax without any capacitor going to ground. Also the different sections of the antenna are joined together by some 2pF capacitors that I also bridged by solder a jumper over them.

The SWR are terrible and it's useless as a transmit antenna but reception improved tremendously in all bands compared to the stock antenna. It works better at VHF air than a Sirio 5/8 GP tuned to 125MHz. It's easy to restore it to stock as everything is still in the antenna and only needs to be soldered back.

/Ubbe

 

[mmckenna]

Well, it turns out the vertical IS DC grounded....

OK, that's not uncommon. Many are, depending on the design.

Different antenna types perform differently. Gain isn't the only answer. Sometimes different radiation patterns work better than others.

Or, different cable lengths. Different coaxial cable types. Different connector performance. Different locations.