Stealth Attic Antennas for DX and Contesting?

  

Facts and Myths  

  

I have heard all the excuses from hams about antennas and housing restrictions.   This page gives some answers - from my perspective - of attic and stealth antennas hoping to set the record straight.

  

Operators and Observers

The fact is most guys who talk about attic antenna limitations are not serious to operate.  The lack of an external antenna is really an excuse for not taking action.  Many of these same hams would have other excuses, if it were not for the antenna, it would be something else.

The world is divided into operators and observers.  Operators find a way to get on the air.  Maybe they guest-op at friend or club site. Or maybe they homebrew their attic to antenna farm nirvana. And anything in-between.  

On the other hand, we have observers. Observers are good guys with tickets, but otherwise inactive.  A lot of observers tell me they want to get on the air  - but they are well equipped with many excuses as to why they are unable too. It's unfortunate, because a prohibition against external antennas is not much of a prohibition - to the motivated operator.  

An example of an operator is John, AI4FR.  Here's a guy that operates with a 102' G5RV at 18' - and a half-wave vertical dipole in a side-yard tree - about 20' at the TOP of the antenna - covering 10m.  At one point that G5RV was laying on top of a hedge row. John averages more than 10K QSO per year running barefoot.  He's got about 10 variations of DXCC on his LotW page. Check out his QRZ.COM page here:  http://www.qrz.com/db/ai4fr

  

Simulation

A determined operator, with an attic of nice size on a 2-story house, really has a wonderful platform for which to launch a great set of antennas. Simulation is a great tool for the attic antenna farmer because it will help you to understand what will NOT work.  There are a lot of reasons that an antenna may not work as you expect, but simulation will help you to know what kind of configurations have a good chance TO work.

A word of caution with simulation - and this is coming from a BIG fan of simulation - if the antenna is around nothing else metallic, and is above the ground a long way, then the model and the simulation should be pretty close with things like end effect and wire insulation largely responsible for the differences. 

But as you move the physical antenna structure closer to things (like houses, attics, or other metal things), the model simply cannot comprehend all the significant interplay with all this other stuff around.  And attempts to adequately model these other things is as error filled as not having them in the model at all.

So build the antenna off of your simulation, but keep in mind, it's not going to be the same thing in reality.  But it is about the best that can be done under the circumstances.

   

Modeling is not perfect - how do I model an attic?

In any antenna installation, a line needs to be drawn between the details that can be modeled and those for which you have no control over.  An example of that would be trying to model the impact of roofing materials, for example.  In my opinion, if the structure is physically large vs. the frequencies you are dealing with (for example - a stove pipe vent which may extend through the center of the house and out into the roof top vent), ignore it.  And for things that are of unknown RF impact - they cannot be reasonably modeled.  

At my house, one end of the attic has stucco with a wire frame under it.  The frame is not grounded and I'm not sure what the effect is with respect to RF energy.  Measurements on reversible arrays with field strength measurement on each end of the house shows similar field strengths suggesting that, at HF, this frame is either non-resonant and essentially transparent, or it's absorbing and then re-radiating the energy absorbed.  These factors serve as loss points, I am sure, but because I cannot control them, I just ignore them.

Conclusion:  Model what you can, to the best you extent you can.  And space your antennas away from the structure where possible.  And the model will generally work out for you as a good design tool.

  

Best advice

My #1 advice to guys working on attic antennas is to view the project as an iterative experiment.  You won't know exactly what will work the best until you try to build the first antenna.  And that experience will lead you to take new paths; consider new ideas.  I'm on the 4th generation of antennas now at my current QTH - and each generation is higher in performance and capability than the one before it. 

  

Fair Winds and Blue Skys - 100% of the Time!

There is an upside for an interior antenna that is pretty significant.  If you love to fiddle with antennas, the attic provides a great environment because you don't need to be worried with the typical out-of-doors weather factors.  And you can work on your antennas when your friends are "grounded" due to weather!  That's great especially when the tornadoes or ice storms hit - I have the choice of playing with my antenna in a dry and not very cold place (compared to the outside winter time) - or getting on the radio and making some Q's. 

  

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Opinions - Myth Busting and Attic Antennas

Here's a list of some of the most common concerns expressed and some observations from my experience. None of these comments should be interpreted as anything other than my personal, non-professional opinion:

  

"Performance is much less than full sized antennas"

Depending on the attic materials used, this can be true.  But as a general rule, the impact is pretty small.  Dry wood is an excellent (not perfect, but very good) insulator.  Consider that many hams use trees for antenna support yet you don't see many articles talking about the RF absorption of these water-based structures!  

Conclusion:  There is some performance impact - but the loss is not significant.  

  

"RFI problems are too great"

There are a few problem areas for the attic antenna farmer and RFI in the house is one.  But this is a pretty minimal problem with the high use rate of cable television feeds and 2.4 GHz or higher wireless hand phones. 

Antennas close to the house, inside or out, extra care needs to be taken to keep the RF off of feed and control lines and that generally means investment in some quality type-31 ferrites and proper wire dress.  NOTE:  TYPE OF FERRITE MATERIAL USED IS VERY IMPORTANT.  

  

RFI problems tend to be worse the lower in frequency and higher in power you go.  But compared to the situation just 10 years earlier, the march of technology certainly makes this a much easier challenge to solve than before.

Resonant antennas have far less RFI issues than antennas relying on shack-based matching solutions.  This is often a tough requirement but in difficult cases, making the antenna naturally resonant at the frequency of operation may solve a lot of your RFI problems and lead to a much lower spousal QRM level.  :)

  

The undisputed master of all things RFI is Jim K9YC.  Must-read resources include these two:

For the state of the art in coax trap construction, see:

http://www.audiosystemsgroup.com/CoaxChokesPPT.pdf

And for a general primer on RFI topics:

http://www.audiosystemsgroup.com/RFI-Ham.pdf

  

Generally speaking, there are one or two wires connecting a device which tend to attract the most RF and - if you can isolate the line - apply your ferrite medicine on those wires alone.  Here with 10 bands of operation, testing every permutation is exhausting - especially if you have several items needing a check out. 

My solution is to take the shot-gun approach - and ferrite each and every wire going in/out of the thing.  An example of my AV receiver is show here at the right.  The disadvantage of this solution is that it's expensive from an investment in ferrites standpoint.  The advantage is in coverage against the many permutations of band and gadget configuration. With more bands of operation and a complicated device like the modern AV switching unit - it's hard to test each combination to be sure you have isolated the right lines - and then isolated them enough.  By covering them all, you provide a level of insurance against future combinations.  And of course, the new combination that you did not test - will work to be the one which gives you problems on the opening day of your favorite contest or when that famous DX-pedition reef operation finally hears you.

Conclusion:  RFI in the house is a challenge, but with some ferrites it's easy to contain

Footnote:  In my experience, operation on 160m/80m and 15m is the most problematic but only in the higher power level case.  Solution?  Put a ferrite on anything sensitive (meaning expensive) or that is making funny noises that it should not when you key down.  Oh, and generally speaking, ALL powered computer speakers are trouble...

  

"Houses are filled with noise sources"

This one, in the modern era, is in fact true.  And tracking down these noise sources is time consuming.  The normal method of tracking down RF sources is to power off the house mains at the switch box - while running your rig from a battery.  Scan each band and that provides the baseline.  Then activate one floor (or room) at a time, comparing your baseline noise sources for anything that's come up.  From there, it's a matter of tracking down the specific noise source.

The most common noise sources are power-supply related.  Laptop power supplies are generally not the problem - it's the aftermarket or eBay-specials I have picked up that were problematic.  

Other sources of problems are the large appliances - I have a 5-year-old washing machine that creates wide band noise bars about every 40 KHz from 80-15m!  Ferrites did not help.  Something about that washing machine makes it a FB noise source.  The solution here was coordination with the XYL on washing times vs. contest times.  Unfortunately, the compromise tends to be "after your contest is done, you can wash the clothes!"  The price for harmonious contest hours is steep indeed...

  

"Switch to our broadband phone/television/Internet service for only $50/mo!"

For this kind of teamed service at this QTH, there are 2 cable options and the U-verse service from ATT.  I've had both the cable services and they work without issue - any band, any power - no drop-outs or other troubles.  I can't speak to the many other services available - but I can offer some caution for the U-Verse product from ATT.

U-Verse is a broad-band product which operates in frequencies as high as the 40m ham band.  It's a "smart" technology in that the adapter box will scan the spectrum and not use parts of the spectrum where it detects interference.  However, there is a problem for the ham with this interference detection.  It's setup to address specifically steady-state interferences like a BC station.  Intermittent types of interference, like CW, cause the box to reset repeatedly.  That results is continuous service outages for the user when you are transmitting.

No amount of ferrite or other bypassing works - because this is not a common mode issue.  The box works as a receiver with coverage from near DC up to about 6 MHz.  It is especially sensitive below 3.7 MHz.  Which means you can have a CW op running on 80m who has U-Verse problems - and his neighbor - a SSB op running on 80m who has no U-Verse problems!

ATT is aware of this issue but in my experience, it's one of these business decisions where the cost of addressing the issue is greater than the customer complaint level.  As of 2010, there was no improvement plan that I was able to discover.  Which leaves traditional AT DSL service as one option for Internet (DSL is very stable in RF environments) with the traditional telephone connection.  And using a non-ATT television cable provider.

My advice - avoid the headache.  If you have antennas close to your house - think cable solutions.

  

"RF levels are dangerous to the people in the house"

The FCC, as part of every license grant or renewal requires amateurs to perform an assessment of RF levels called Maximum Permissible Exposure (MPE).  Many links are available to explain these requirements but the most simple I have found is this tool, thanks to Jon, W0JEC:

http://www.paulbunyan.net/users/jcrisman/index.html

Even with amplification, the exposure level for inhabitants is well below threshold in most cases.  The MPE Calculator tool makes it very easy to determine the exposure limits.

Conclusion:  Even at the legal power limits, RF levels are almost always within the "safe" operation region.

  

"Antennas in the attic are a fire hazard"

This is one of my most favorite excuses because the factual case of a fire hazard is nearly impossible to make.  For the most common type of antenna, the dipole, the high voltage point is the ends of the antenna.  And at roughly 30Kv/inch, it does not take much of a gap to guarantee no spark could ever be created.

But spark to where?  Critics fail to consider that electricity requires 2 conductors to flow.  The end of an antenna may be a high voltage potential point, but where is that spark going to jump too exactly?  Dry wood has an extremely high resistance value, even at RF.  So even with a bare wire connected directly to that wood, the return path is essentially infinite because the return loop is very far removed. Earth ground is located 10-40' below - and the other end of the antenna may be 40-120' away! 

The worst case I have seen is a modeled scenario where a 160m loading coil was being used with a 1500W amp feed. The calculated voltage across the coil was around 25,000V. But the physical length of the coil was about 18 inches. To get a spark, wires would need to be run from each end and held in close proximity. And I think you would need to have some wood adjacent to the path - but not blocking the path - so that the spark would somehow create enough plasma to catch the wood. This situation is so contrived as to simply be unbelievable. My point of all this is not that it's impossible.  But rather, it's extremely difficult to do - even if generating a spark IS your objective!

Now this is NOT a comprehensive security blanket for ignoring good engineering and safety practices.  And to the extent local regulations or perhaps insurance company considerations exist, you may be required to expand your precautions.

When building attic antennas we want to keep the wires away from the wood structure, especially at the ends, because of the capacitive detuning effect that comes from the proximity.  This is good antenna building logic.  AND it has the side benefit of essentially fire proofing your installation even if you feel the need to kick on that 4CX15,000!

Now if you know of a case where a professional investigation concluded that the ham radio antenna was the cause of an attic fire, please send me information on that.  I would love to feature that case here on the site!

Conclusion:  It is theoretically possible to create a fire in the attic from your ham radio antenna - but the conditions required for that to work require purpose and luck making this not a realistic concern for the typical attic farmer. Falling through the roof is a much more likely scenario!

    

"My house has a metal roof and is covered with stucco on the sides"

OK - I give up on that one.  :)  

  

"The attic is hot."

If you need to be told - "Don't climb your outside tower in the ice storm."  Then you should also appreciate the advice "don't build attic antennas in the heat of the summer when the attic is 50C."  Attic work requires the physical skill needed to climb rafters and not fall through the ceiling.  You know very well if you qualify for that work or not.   

  

"What if I fall?"

Attic antenna farming requires the ability to negotiate rafters and keep your feet on the flooring studs.  If you slip and fall, your foot will easily go through that sheetrock ceiling and the XYL will surely not be very understanding...  

  

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