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Analog Angle Blog

My Antenna Dilemma: Preamp or Passive?

Engineering is largely about tradeoffs and constraints. Sometimes, the issues are clouded and the choices are fuzzy, but occasionally, the choice has a fairly clear split.

I had one of the latter situations with my TV antenna. I'm one of those folks who rely solely on free, over-the-air (OTA) broadcasts. (The reason for this OTA life is a subject for another discussion.)

When the US switched over to digital TV a few years back, I kept using the UHF bow-tie antenna in the attic:

Its balanced output goes through a 300Ω/75Ω impedance transformer (a standard balun you can get for about a $1) and feeds about 100 feet (30 meters) of coax to the TV. Even though the antenna has the wrong electrical size for the frequencies assigned to DTV, it has been working pretty well, so I left it as is.

But one channel's signal is marginal, and I get break-up when the leaves are in full bloom or there's heavy rain — I suspect it's due to RF absorption. The problem is that while analog TV degrades somewhat gracefully with decreasing signal, it's quite different with digital TV: You get “all or (almost) nothing at all,” to paraphrase the old song. The decoded image “falls off a cliff” and freezes, breaks up, or goes blank when the received signal strength falls below some threshold.

So, I have two choices: either replace the bow-tie antenna with an antenna with a preamp, or put up a bigger, better passive antenna. Since this is an in-the-attic installation with plenty of room, I am not constrained by appearance or space, so I can take that out of my mental equation.

What are the parameters to consider? There are four: received signal strength at the antenna, SNR at the antenna, signal strength at the TV, and SNR at the TV. They differ at the ends due to loss in the coax run, plus added cable noise.

Choice No. 1 : Adding an antenna with a built-in preamplifier should help. It won't improve received SNR at the antenna itself, but it will help deliver a signal with much better SNR to the TV. Cost is around $30-50, and from what I can tell, they are omnidirectional or at least bidirectional. That's good, because the signals I want to get are from two major cities, and I am midway between them.

There's also a small AC/DC “wall wart” that would plug into the attic AC outlet to power the preamp. Here I am a little concerned, as those basic, inexpensive adapters don't have the high MTTF (mean time to failure), and I suspect I would only get a year or so of use in a hot attic. Admittedly, replacing one is not a big deal; it’s just another thing to do.

For Choice No. 2 , I could build a bigger antenna with some additional gain. There are plenty of plans available online for these antennas, and they all seem to use the same basic design (see here for a typical one). The SNR of the captured signal should increase, but the losses in the coax might still be a problem. Some designs add a screen or solid reflector spaced behind to add additional directional gain, which may actually be a bad thing here as I am looking for both front and backside gain. Cost is next-to-nothing, and reliability is not an issue at all. As for a power supply — none needed, of course.

So, that's the choice I face, and the pros and cons are fairly clear. The preamp version adds gain at the head end, and should reduce the effects of the coax run. The passive version is much lower cost, and extremely reliable, but may not improve the far-end signal strength and SNR numbers enough.

Since I don't have the instrumentation or inclination to make measurements, any analysis or modeling I do would be a wild guess. When faced with such a dilemma, I'll choose the simpler option first and try that passive antenna. Sometimes, despite our affinity for “electronics,” a passive solution can be the best, or at least worth a first try. If that doesn't work, well, hey, it will have been fun at no cost, and with no risk. (See: No pain, and some gain: how “more metal” saved my RF link.)

Have you ever had a similar A versus B choice? How did you decide what to do?

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25 comments on “My Antenna Dilemma: Preamp or Passive?

  1. Brad Albing
    June 6, 2013

    Hi Bill – I noticed you said your bowtie antenna is the wrong size. Did you mean it's not very good at receiving VHF (channels 2-13)? I would expect it would be pretty dreadful in the VHF band unless you're close to the transmitters.

  2. eafpres
    June 6, 2013

    Hi Bill.  The choice of A vs B in your case might depend on a couple of factors realted to the received signal orientation and pattern of your antenna.  You mention you are looking for both front and back side gain.  This leads me to beleive you are receiving from multiple broadcast towers?

    If you go with a bigger antenna, there is a tradeoff somewhere in the pattern to get more gain.  It has to have higher directivity which means the pattern either is less omnidirectional or it has a narrower beam width somewhere or it becomes asymmetric or “all of the above”.  If you know where the towers are, you could construct a more directional antenna and aim it.  However, if there are two in relatively opposite directions, it is a little harder.  

    I once worked on a design using a patch antenna where we reduced the ground plane dimensions to the same dimensions of the patch.  It was nicely bi-directional–it actually had two ~5 dBi lobes facing in opposite directions.  You might be able to pull something like that off if you put in a large reflector (i.e. sheet of metal, or foil) and used two antenna elements feeding into a coupler.  The elements would need to be about 1/4 wavelength away from the reflector.  Without any test gear though, you have no way to know if it is really mismatched.

    That problem is there with your current plan–how will you know what impedance your new antenna is?  Are you going with a design that says it is 300 ohms?  That might work, or could be wildly wrong.

    For all the above complications, I think the preamp would be a higher probabiity path.

  3. Scott Elder
    June 6, 2013

    A lot can be learned by having someone watch the screen while someone else stands next to or touches the antenna.

     

  4. Bill_Jaffa
    June 6, 2013

    Brad, it's the standard UHF bowtie (approx 470 to 800 MHz) which was used in conjunction with rabbit ears for VHF (approx 55 to 85 MHz and 175 to 215 MHz).

    The digital TV frequencies are, I believe, approx 470 to 690 MHz and sometimes 174 to 210 MHz. So the UHF bow-tie may so OK, sort of, for some DTV (but could be sized better) and not OK for the other DTV group.

  5. Bill_Jaffa
    June 6, 2013

    I'll probably start with passive, there's something appealing to me about a non-electronic, never-fail solution (once it's set up). Plus, my target stations are on either side of me, so if I use an antenna that has modest forward gain and backside gain, and no gain in the sidelobes, I should be OK.

    As for impedance, for the antenna designs I have seen (see the link) it seems like it should be 300 ohms, it uses a series/parallel combination of multiple 300 ohm folded dipoles–a standard configuration for antennas. What's attractive here is that the physical dimensions are so small, and the antenna is going to be indoors, so construction can be with stiff wires, don't need tubing elements, supports, or weatherizing.

  6. Brad Albing
    June 6, 2013

    So there is no longer transmissions in the VHF low-band (the old channels 2-6)?

  7. Brad Albing
    June 6, 2013

    In fact, sometimes you can just pay one of your kids to stand in the correct spot to improve reception.

  8. Brad Albing
    June 6, 2013

    That link that goes to the Popular Mechanics site shows what initially looks like a simple, straightforward antenna that you could put together pretty quick and pretty cheap – except it's lacking a few critical dimensions. Even after some comments were left to this point – no response from the author.

  9. Netcrawl
    June 7, 2013

    Today's antennas has signal-strength meters built-in, this was designed to help aim an antenna properly or to get a better reception. Its up to the you on how you position the antenna, once you have the station locked-in then you need to cross-reference the chart, make sure its coming from the main broadcast station.

     

  10. Netcrawl
    June 7, 2013

    @Brad you're right, you need company, someone must watch while the other touch the antenna, spotting for better reception is  a daunting task, the slightest adjustment can make a major difference in signal strength, yes I have done this before in our home. 

     

  11. DEREK.KOONCE
    June 7, 2013

    I like the passive approach first since, as you say, the active device may have issues in the hot attic. The other approach is to confirm the direction of the antenna. The antenna you have pictured only has one direction possible when strapped to the rafter.

  12. Bill_Jaffa
    June 7, 2013

    The print version had dimensions–why they left them off the online one, I don't know. But there are many almost-identical ones on the web, plus YouTube videos, which have dimensions.

  13. Brad Albing
    June 7, 2013

    @Bill >>The print version had dimensions–why they left them off the online one, I don't know . Probably some editor screwed something up. You know, I'm sure, that editors are often idiots. But there are many almost-identical ones on the web, plus YouTube videos, which have dimensions . Almost too much info on the web – pro'ly take a while to wade thru it all and find something (or a consensus of some things) that I could trust. Just sayin'….

  14. Brad Albing
    June 7, 2013

    @Derek >> The antenna you have pictured only has one direction possible when strapped to the rafter . What Bill didn't mention is that he has his house on a turntable that he bought surplus from a railroad round house that was being decommissioned – it's like a giant lazy-susan (no offense intended to my female coworker named Susan). So he just rotates his house until he sees a signal maximum.

  15. Davidled
    June 9, 2013

    I think that Antenna might have a good impedance matching whether to use passive or to use pre-amp. As shown in the picture, there is a brown strip line effecting impedance.  Also, size of antenna might be bigger than that shown in the picture. Therefore after tuning these factor, the modified antenna might have a better performance.

  16. WKetel
    June 13, 2013

    A simple set of fabricated co-linear antennas like the one described in the link would be a good choice for in the attic, followed by the best low-loss balun available, and connected with a very good grade of low loss coaxial cable, would provide the best compromise. No amplifier to die or go into oscillation, no wall wart to waste power and then fail, and a typical time between failures in the tens of years, or better if it is kept out of harms way.

  17. Bill_Jaffa
    June 13, 2013

    I agree–and it would be lowest cost and most fun to “experiment” with. Sure, quality low-loss coax is more expensive that junky coax, but it would be a good idea to use it regardless of whether I go active or passive–so that is not a cost difference in one versus the other,

  18. Brad Albing
    June 14, 2013

    OK – Bill, I await further reporting on your research regarding the co-linear antenna array. Otherwise, I'll be forced to do my own experimenting.

  19. Brad Albing
    June 24, 2013

    Finally checked this myself – looks like pretty much nothing on the VHF low-band now.

  20. Brad Albing
    June 24, 2013

    I did track down more info than I could shake a stick at regarding those V-array configurations. Oddly, none of the descriptions I read spoke to these two issues of interest:

    1. In a 4-array, why is polarity swapped on the top and bottom antenna sections, but the middle two are operated in phase?

    2. In some versions, a reflector is placed behind the array (behind being arbitrary – there is no behind until you put a reflector there). No one states what the spacing should be between the reflector and the array elements.

    Does anyone here know?

  21. Bill_Jaffa
    June 24, 2013

    Re #1: I'd have to refresh my memory, starting with the ARRL Antenna Handbook (lots of practical information there); undoubedly it is some sort of phasing thing. (Antennas and arrays always seemed like a lot of mysterious magic to me, frankly.) But some of today's EM field-analyis tools can do an amazing job of modeling and assessing antenna performance–hello, anyone out there willing to model this?

    Re #2: I assume it's worth starting with a lambda/4 spacing. The nice thing about this design it that it is easy try various postions and adjust and try again. And since it is indoors, there is no need to climb on the roof, and the elements and reflector don;t hav to be heavy-duty materials!

  22. eafpres
    June 24, 2013

    The best spacing is 1/2 wavelength at the center frequency.  A simple way to consider this is to first think about wavefronts leaving the antenna heading towards the reflector, and ones leaving the antenna in the opposite direction.  If the spacing is 1/2 lambda, then the reflected waves arrive in phase with the waves going the other way, and have constructive interference, increasing the gain.

  23. Brad Albing
    June 25, 2013

    @eafpres – I'll try the λ/2 spacing and see what happens.

  24. Brad Albing
    June 25, 2013

    @Bill – Me too regarding the mysterious magic. More complicated than a dipole with no reflector or director elements around it – else I'm lost.

  25. PCR
    June 30, 2013

    True Brad, It is a wasting of time to repair the antenna. 

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