Antenna on the roof or in the attic ?

Discuss commercial and home made antennas.
James_from_France
Posts: 39
Joined: Sat May 13, 2017 4:02 pm

Antenna on the roof or in the attic ?

Post by James_from_France » Sat May 13, 2017 4:30 pm

Hello from France,

For the moment, my antenna (Centerfire Deluxe Discone) is in the attic. At each end of the attic, there's another house and on the sides of the attic there's the tiles of the roof with no insulation.
Would it make a lot of difference if I put my antenna on the roof ?
For the moment, I'm using a cheap LNA from Aliexpress, does the LNA4ALL make a lot of difference in reception ?

Thanks for your help,

Regards,

James

snn47
Posts: 192
Joined: Tue Dec 27, 2016 11:00 pm

Re: Antenna on the roof or in the attic ?

Post by snn47 » Sat May 13, 2017 5:57 pm

From the specs your discone has a coverage from 30-1300 MHz, which means you normally receive only within Radio Line of Sight (RLOS). Therefore the higher your antenna the better. But it depends on your locations height, if your antenna in the attic/roof is above other obstacles if the height difference is worth the trouble or not.

RLOS in km is defined by the heigths of the transmitting (h tx) and the receiving (h Rx) antenna, for curved earth without terrain or obstacle shielding like mountains or houses, between both.

RLOS [km] = 4.13 * ( sqrt (h tx [m AMSL]) +sqrt (h rx) [m AMSL]))

Rx Receiver
sqrt square root
Tx Tranmitter
AMSL Above mean Sea Level

PS.: Another factort to keep in mind is the transmission line loss of your coax cable.

Kenn
Posts: 51
Joined: Fri Apr 08, 2016 8:43 pm

Re: Antenna on the roof or in the attic ?

Post by Kenn » Sat May 13, 2017 8:40 pm

Nice antenna, I have a similar discone and like it for general purpose.

A few thoughts about your installation:

As snn mentioned VHF & UHF signals are predominately line of sight, so the higher you go the farther out your radio horizon extends.

Attics of course do provide good protection from the elements, but even some non-metal building materials can reduce signals. If nothing else, getting the antenna above & away from metal flashings, wiring, ductwork, etc. can help.

How much additional feed-line will you need? If it's a lot just to gain a few feet the losses might negate the height advantage. Also, the longer the run and the higher the frequency the more critical feed-line losses become. Here's a great resource:

http://www.timesmicrowave.com/calculator/


In the end, theory is great but then there's practice. I've noticed that where I put an antenna is at least as important as how high. ie it works great over the garage but not as well over the living room. I've also had situations where an antenna worked better on a 5' mast than it did on a 10' mast. Go figure! Ya just have to try a few things and see what happens. Make note of signal strengths of a few stations and set it up there and compare. Maybe try at least two spots on your roof before making it perm.


-Kenn


http://ranous.wordpress.com

James_from_France
Posts: 39
Joined: Sat May 13, 2017 4:02 pm

Re: Antenna on the roof or in the attic ?

Post by James_from_France » Sat May 13, 2017 10:03 pm

Hello and thanks for the answers.

I won't need much more coax to go from the attic to the roof. So it shouldn't be a problem. As the roof is quite high, I won't be fitting the antenna myself. In other words, I can't experiment different heights or different positions.
At the moment, the antenna is 'in between' the other houses. If I put it on the roof, it will be over the other houses. Should that make a difference ?
I don't want to put my Rtl-sdr dongle right underneath the antenna with a long USB cable because I also use the same antenna to play with my Baofeng Uv-5r.
Have you tried the LNA4ALL ? Is it as good as everyone says ?
Another question (nothing to do with antennas ) : Does a 'voice' scanner exist for sdr# ? I'm looking for a scanner that only stops on voice signals, not on digital signals .

Thanks again,

Regards,

James.

Kenn
Posts: 51
Joined: Fri Apr 08, 2016 8:43 pm

Re: Antenna on the roof or in the attic ?

Post by Kenn » Sat May 13, 2017 10:30 pm

I'd place bets on there being a noticeable improvement. Go for it! As for the LNA4ALL, we noticed a significant improvement with ADSB. Rough estimate, about 50% improvement in range and more aircraft detected. That's also using an ADSB filter.

Placing the dongle as close to the antenna as possible eliminates feed-line losses, but if the dongle is outdoors it's subject to weather. I'm in the Sonoran desert, I'm pretty sure I'd roast it if I tried that. My feed-line is only 20' of RG6 so losses are minimal anyway.

Dunno about a voice-no-data plug-in for SDR#, that would be handy though.

-K

rtlsdrblog
Site Admin
Posts: 2434
Joined: Mon Nov 19, 2012 11:54 pm

Re: Antenna on the roof or in the attic ?

Post by rtlsdrblog » Sat May 13, 2017 11:51 pm

I think you'd see a good improvement if you improved the line of sight of the antenna. Especially too if your roof shingles are the type with metal braces, or anti-growth copper in them.

The LNA4ALL is a good LNA. But remember that an LNA will make the biggest difference when placed near the antenna. This helps overcome coax losses.

Be aware that an LNA can make things worse if you are experiencing a lot of overload from very strong signals.

snn47
Posts: 192
Joined: Tue Dec 27, 2016 11:00 pm

Re: Antenna on the roof or in the attic ?

Post by snn47 » Sun May 14, 2017 4:10 am

Was Kenn correct you want to primarily receive ADSB, or is your interest more in terrestrial signals? If it is ADSB a Q&D (Quick and Dirty design) using a small 1/4 wave ground plane on a ~5 m fiberglass fisching rod or 2 by 4 piece of wood, is easier to set up and secure e.g. on a balcony, than a large Discone. Before you spend a lot of money on getting the antenna mounted at a spot that may not work well, try a simple mounting as proposed above on different locations for your discone to get a feeling on how well you can receive the different stations on differnt frequencies around you.

Antennas and how to site them best depend on to many factor to give more then general guidelines. The actual real antenna pattern at any location is influenced by many other factors and not just height above or proximity to objects/planes that are conducting or influence electromagnetic waves. Since you have a very broadband receiver/antenna all will vary with frequency/wavelength a lot too.

If you don't have the means to measure more then direct short, then there is mechnically abuse of cable or the connectors (overbent/stretched) during set up of the antenna and later to consider. This will vary the transmissions lines impedance and attenuation from antenna out to stick.

PS.:Paying someone to mount your antenna, connector and cable does not mean it will work well later. Colleagues told me horror stories, when very (expensive) rigid 2" cellflex coax was overbend by 90° or less, e.g. arround corners or when much more flexible RG-213 was stretched forcefully, when it was a bit to short by pulling on the connector or more forcefully using both hands to stetch the cable enough, then to adjust to the building rather by drilling holes in locations in a way that would not damage the cables. :twisted:

James_from_France
Posts: 39
Joined: Sat May 13, 2017 4:02 pm

Re: Antenna on the roof or in the attic ?

Post by James_from_France » Mon May 15, 2017 10:02 am

Hello again.

I use my rtlsdr dongle for a bit of evrything (it's my new toy) : POCSAG, NOAA, listening to planes, ships (I live near the port of Calais in France),... I think that I was a bit disappointed because I expected to hear lots of coversations like when I used to have a CB (Excalibur MK III) in the 80's. These days, everything seems so quiet (or maybe everthing is digital ?).
I'm waiting for the rtl-sdr blog v3, I hope that i will make a difference. I think that I'll also order a LNA4ALL to go with it.
For ADSB, I'm using a 'cantenna' (in the attic), with no LNA. It seems to work quite well, up to 200 miles.

If a decide to put the discone on the roof (with the LNA just underneath in the attic), what is the best coax to use ?

Thanks again for your help.

Regards,
James

rtlsdrblog
Site Admin
Posts: 2434
Joined: Mon Nov 19, 2012 11:54 pm

Re: Antenna on the roof or in the attic ?

Post by rtlsdrblog » Mon May 15, 2017 10:49 am

James_from_France wrote:Hello again.

I use my rtlsdr dongle for a bit of evrything (it's my new toy) : POCSAG, NOAA, listening to planes, ships (I live near the port of Calais in France),... I think that I was a bit disappointed because I expected to hear lots of coversations like when I used to have a CB (Excalibur MK III) in the 80's. These days, everything seems so quiet (or maybe everthing is digital ?).
I'm waiting for the rtl-sdr blog v3, I hope that i will make a difference. I think that I'll also order a LNA4ALL to go with it.
For ADSB, I'm using a 'cantenna' (in the attic), with no LNA. It seems to work quite well, up to 200 miles.

If a decide to put the discone on the roof (with the LNA just underneath in the attic), what is the best coax to use ?

Thanks again for your help.

Regards,
James
CB is mostly dead these days, but it's still possible to catch a conversation. But POCSAG, NOAA, listening to planes, ships should be all fine.

For coax I usually recommend RG6, but if its a short run of only a few meters and/or you're using an LNA RG58/RG59 will be ok.

snn47
Posts: 192
Joined: Tue Dec 27, 2016 11:00 pm

Coax for Antenna on the roof or in the attic ?

Post by snn47 » Mon May 15, 2017 2:17 pm

Check the HAM Bands and you should get a lot of conversations.

For cable you should check for
- do you have many strong (local) that you have consider for very good shielding
- you don't necessarily the ones written above there may be other coax-types (cellflex, flexwell) with similar/better specs available if you order a roll, so check which cable are sold/shipped in/to France compare which give you the best value for starting. When you get more experience you'll properbly will change/upgrade anyhow.

I tried to sumarize all in here "Guidelines for choosing coaxcable for use >1000 MHz" http://www.rtl-sdr.com/forum/viewtopic. ... 1516#p3828

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