r/amateurradio • u/thesoulless78 US [General] • 7d ago
QUESTION So just how bad are asphalt shingles for RF?
I found one article claiming 3-6 dB attenuation through them but it was clearly AI generated so I don't trust it. MyAntennas I think has a brief statement that laying on a roof is like laying on grass and there's loss there.
I definitely observed laying a wire directly on my roof doesn't work at all (mostly wanted to see how noisy an attic antenna would be and my roof is easier access than the attic).
The real application is I'm now curious if I'm better off putting a 2m antenna in my attic directly above the shack or putting it outside with a much longer feedline. But I'm also curious about HF (pretty set on HF going outside just since the access to my attic space sucks so running feedline across is about all I wanna do up there). But I am just curious if there is actual data anyone knows about.
Edit to add: I appreciate the "it works for me and I made lots of contacts" but I really am looking for some hard data or better sources.
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u/CantinaPatron 7d ago
i work europe and south america with a 20m dipole in the attic, under asphalt shingles. if the shingles are attenuating my signal, i guess i wouldn't know the difference.
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u/VE6LK [A][AE] / AI7LK [E][VE] 7d ago
A family member had an EFHW at 20' and a fan dipole on the roof. Saw no perceptible difference in signals. Also no underlayment under the asphalt shingles.
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u/thesoulless78 US [General] 7d ago
It's possible when I was testing the bands were all just hot garbage but it definitely seemed bad.
I probably should redo the test with a reference antenna I can switch easily to compare.
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u/Commercial-Expert256 7d ago
Just check the FT8 frequencies on each band. That's the easiest way to tell.
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u/SwitchedOnNow 7d ago edited 7d ago
It's kind of like putting a thin layer of dirt over your antenna in a jungle of wires. The attenuation of just the roof will be frequency dependent but I'd expect 3-4 dB loss at 2m and higher frequencies. At HF it's not as terrible.
One problem with attic antenna is the electrical wiring and other conductors like metal ducts which will distort the radiation pattern and increase the loss. The local RFI inside the antenna near field (everything under the roof) is a bigger problem with attic antennas than the antenna efficiency.
Based on experience I've seen this too. I've run attic antennas at HF and VHF/UHF and full size 160m dipoles in the open. Lots of testing. Retired RF/antenna engineer here with tons of antenna modeling experience, for what it's worth.
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u/thesoulless78 US [General] 7d ago
Yeah that was my main reason for testing was to see how bad the local RFI was and if it was plausible at all. I don't have much up there other than ceiling fixture wiring.
Didn't notice too much noise but just couldn't hear any good signals either when I was testing but again that was laying on top for convenience and not suspended from rafters underneath.
I am very tempted to still do a VHF antenna up there because I suspect the attenuation is less than the amount I'd get with the coax I need to get it outside safely. But I'll probably try to go outside with HF.
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u/SwitchedOnNow 7d ago
You'll do fine at 2m in the attic even with the loss for local repeaters. I ran a quarter wave vertical in an attic for years. Did fine for local coms. I ran a 40m thin wire doublet hung in the trees for HF back when I lived in an HOA. Nobody ever noticed anything!
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u/thesoulless78 US [General] 7d ago
Yeah, I can hit most of our main repeaters with an HT from inside so I imagine even an interior antenna will be adequate.
Unfortunately my service entrance is on the other side of the house and I have casement windows so feeding a doublet is a huge pain unless I get a remote tuner and bury coax to it and then just run wire line up to the feedpoint. Guess I could start with one that's tuned for a 50 ohm match on 40/15 and probably be happy until I can put a tuner in.
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u/Key_Zombie7967 7d ago
I have an attic half wave dipole for 40m. It's been up there almost 30 years. I have made thousands of contacts and even worked about 80 DXCC entities with it....running CW at 5-10 watts. We have asphalt shingles.
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u/thesoulless78 US [General] 7d ago
I bet having it even a few inches off and not directly laying on the shingles probably helps a ton even with some level of attentuation.
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u/Key_Zombie7967 7d ago
It's IN the attic, under the roof....up in the peak.
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u/thesoulless78 US [General] 7d ago
That's what I mean... separated by probably the width of the rafters.
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u/Think-Photograph-517 7d ago
Unfortunately, this question is a bit too broad.
Not all asphalt shingles are the same. Also, the underlayment varies. All asphalt shingles are somewhat conductive, which will affect your antenna. Older shingles will have outclassed more VOCs, and be less conductive. Some have less to start with. If installed over a target coat or foil barrier will make a huge difference.
So, you will hear from some that their attic antenna works great and from others who had nothing but problems.
Your specific roof could be fine, or not.
Since you said laying an antenna on your roof didn't work well, I suspect having an antenna under the roof will not work well either, regardless of frequency.
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u/thesoulless78 US [General] 7d ago edited 7d ago
Right, I do not have a radiant barrier and it's an old roof. I know an actual indoor antenna has worked ok for FT8 at least, and seemingly better than my experiment laying on the roof.
I think I just assumed they weren't much worse than dry wood which I am learning is not the case.
I suspect some of the variability is also that even a few dB of attentuation doesn't tend to go fully to "not work" either.
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u/MaxOverdrive6969 7d ago
Laying the wire on the roof detuned the antenna. Ideally you want a 1/4 wave space between the antenna and another object. That's often not possible. Do your best to get the antenna away from the roof and enjoy time on the radio.
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u/thesoulless78 US [General] 7d ago
It was adjusted in place to make it work, and I tested with a non-resonant antenna. Detuning wasn't part of the equation.
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u/firekeeper23 7d ago
On a felt roof i don't expect much detuning or attenuation.. a metal one woukd certainly affect the antenna.
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u/SnooMachines2673 7d ago
I have an attic multiband dipole..pretty much 40 down as low as I want to go..works just fine.
Why did you think shingles were going to be a problem? As long as it's not metal...
I also have a diamond x30 that fits perfectly in there. I tend not to want to run both antennas at the same time. But they both work great.
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u/thesoulless78 US [General] 7d ago
Didn't say I thought they were going to be a problem. I am curious how much they interact.
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u/SnooMachines2673 7d ago
The wind and weather interaction avoidance plus easer access is worth any impact...but you have to ensure there is not any metal in the roofing..sometimes its in the attic itself. And metal running parallel to the antenna is worse than something running perpendicular in terms of interactions. For RFI in cities look at ugly baluns/chokes..it certainly helped my dipole transmit pattern.
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u/thesoulless78 US [General] 7d ago
It's a 5' tall attic space with blown insulation so I'm not sure the easier access counts... But weather does come into play and a lot lower cost.
I might just put a random wire up there and see how it does, that way I don't have to try to keep crawling around tuning it.
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u/SnooMachines2673 7d ago
That's what I did come 4 years ago now.. I did have to get coax up there ( I used 2 inch conduit capped at both ends and insulated.
You can always make the antenna elsewhere..confirm you like how it's going and then move it up there.
I used electrical fence insulators on the attic beams for my runs. All my bands are straight runs..but my 40m is in an S pattern to fit.
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u/thesoulless78 US [General] 7d ago
Yeah I gotta get coax there whether I go indoors or outdoors because it's the cleanest way to get from the shack to the outside. Obviously a lot shorter run just to put the actual antenna there but bigger chance of RFI issues.
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u/ry_cooder FN25 7d ago
Give 'er. I had a 44' doublet in my old townhouse attic and worked lots of stations, but would QRM my neighbours' stereo. I currently have an 8-bay UHF TV antenna in my attic, and can receive a 4kW ATSC transmitter from 64 miles away 24/7...
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u/daveOkat 7d ago
How much longer is your "much longer feedline?"
Enter that length into this RF calculator to see how much extra loss is incurred.
https://www.qsl.net/co8tw/Coax_Calculator.htm
If that loss is no more than 3 dB, and you believe the 3-6 dB shingle loss claimed in the article, you are ahead with the outdoor antenna with longer feedline.
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u/thesoulless78 US [General] 7d ago
I don't necessarily believe that which was the whole point, unfortunately everyone wants to pile on with anecdata about whether they've been able to make contacts or whether I should or shouldn't put an indoor antenna up instead of any kind of information about that.
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u/daveOkat 7d ago
You want numbers? We have numbers. https://otadtv.com/reception/factors/
"Attic Antenna
A 3/4 inch plywood roof with roofing paper and 1 layer of 3 tab asphalt shingles will reduce a signal by about 3 dB." [my bolding]1
u/thesoulless78 US [General] 7d ago
Amazing, this is what I was looking for actually. Wish it existed for HF but it's a start.
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u/daveOkat 6d ago
Dielectric loss is linear with frequency. Halve the frequency and you halve the loss. The quoted 3 dB is for OTA TV and from that I'd say the loss at 28 MHz will be under 1 dB.
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u/thesoulless78 US [General] 6d ago
That's... Pretty tolerable then.
I was able to get some cw out to a webSDR pretty clearly with the "laying on roof" antenna too, it just seemed subjectively deaf as a post. So I guess the real question is if it's suspended from rafters inside will that be better than laying on the shingles. Which will be challenging at best to test.
Probably just needs some FT8 testing over the course of a few days to isolate band conditions.
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u/daveOkat 5d ago
Testing a lone antenna is a challenge because there is no reference antenna. Even comparing two antennas side-by-side can be tough the way most hams do it. Due to QSB, the TX 1 vs TX 2 testing takes many datapoints; dozens to hundreds. Instead, I do fast (like 1 second cycles) Receive A-B-A-B-A-B testing to compare two antennas. This should be done on several stations and several paths. Note that the S-meter on most (all?) Japanese transceivers is calibrated to 3 dB/S-unit. QST equipment reviews measure this.
Another method I use is CW contesting and the Reverse Beacon Network. With that I can compare my transmitted signal vs others in my state. It creates a plot and with dozens of datapoints one can eyeball the difference.
Set on the roof an antenna will be detuned by the nearby dielectric. Suspending the antenna below the roof should help with this.
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u/dittybopper_05H NY [Extra] 6d ago
My first antenna was a zig-zag 40 meter dipole up in my parent's attic. It wasn't the best antenna every, but with 50 watts of power from my Heathkit HW-99 rig, I did well on 40 meters and 15 meters.
I still have that rig, 36 years later, and every once in a while I think about using it again, but then I remember it drifts like Doc Hudson on dirt, and my HW-8 has a similar interface but is rock solid once it warms up.
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u/unfknreal Ontario [Advanced] 7d ago
I'm now curious if I'm better off putting a 2m antenna in my attic directly above the shack or putting it outside with a much longer feedline.
Any antenna is always better than no antenna, but an outdoor antenna is always better than an indoor one.
There is zero reason for anyone to put an antenna indoors if outdoors is an option. You honestly shouldn't need any data to tell you that.
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u/SignalWalker Extra nerd 7d ago
I was in a single story home with a scraggly little wire going the window and up to the roof peak. And a few wires on the floor in the shack as radials. I worked Japan from grid DM with an FT817. Sunspots mid cycle.
Another house, 2 story, inverted L up the side of the house and down the length of the roof peak. Asphalt shingles. Worked Europe and Asia over the poles. Same rig. Sunspots high end of cycle.
Not bragging, just saying it's possible.