If you haven't learned to model antennas you should.  It's lot easier to model than build and it saves money and time in the long run.  I like MMANA-GAL and I'm just starting with 4nec2 but mostly I run Unix, so that leaves just NEC2.  EZNEC is supposedly easy but the free version is too limited for much more than a dipole.  Some ARRL stuff comes with a version of EZNEC, I don't know how limited that is.

One thing about perimeter loops like this is that they're mostly NVIS antennas which means they beam your signal straight up.  If you can put one side of the loop above the other it's at least bidirectional.  The signal comes out at right angles to the plane of the loop.

Modelling yourself lets you play around with ideas.  I did a perimeter loop in an apartment I had once and it was about useless at least for listening.  I've since modeled loops around the eaves of houses, triangular loops in gable ends.  I have one in the attic now that runs along the ridge with the lower side just above the attic floor, fed in the middle of the lower side with balanced line.  Picks up trash from TVs, computers, etc. in the house but it's not bad late at night.  I don't worry much about lightning with it, which is why I put it up.

Trying to put antennas on them is one disadvantage of metal roofing.  If you can get a single mast up you could do a triangular loop with the vertex up on the mast and the lower side a foot or so above the roof.  Or two masts, but the metal is really a problem otherwise.  What about fiberglass roofing?

Are there trees near enough to be of any use?

  Alan, ab1jx

Beaming up at the zenith is fine for a beginner like me. There'll be some tilt to the earth with seasonal changes, but there are hundreds of radio sources in almost any direction, especially in the milky way.  Other than seasonal changes though I think a fixed antenna gets pretty much the same thing every day, so eventually you go to 1 GHz and up and use dishes that you can tilt.  You can actually map the sky by just moving from the south horizon to the zenith to the north horizon and waiting for the earth to rotate.  A narrow beam in declination (north-south) provides some selectivity.

Both MMana-Gal and NEC2 give this antenna 9 - 11 dbi gain, but I don't know how to get a "real" ground in NEC2 yet so I think that's figured with a ground plane.  Mmana's definitely using real ground, not perfect.  What I've got up on one side now is a fat dipole that's at http://ab1jx.webs.com/toys/jove/antennas/fat8/index.html but the other side's a fan dipole with less gain and I think the imbalance is causing trouble.  I've been pretty happy with it, it's just a little unorthodox to describe to someone and I don't have the materials to build a matching one for the east side.

I just checked again: by MMANA-GAL a 3 element has 7.82 dbi gain, a 4 element has 9.21, I don't have space for longer than 4 elements.  End-end spacing used was 50 mm.  This is dbi not dbd.  I'm also sort of a fan of the coaxial collinear but those are about impossible to model.  Some people use a pair of dipoles, but usually with preamps.  LOFAR in the Netherlands uses a bunch of 6 meter inverted vees with preamps to cover 20 - 80 MHz. The multiple antennas aren't for gain, they're for different vantage points.
http://www.astron.nl/radio-observatory/astronomers/technical-information/antennae/antennae-description
And they're spread across Europe too, all linked together.

OK, I'll use a 13:4 turns ratio to try for better efficiency.  I think I can make each antenna out of 2 pieces of wire about 31 feet long with the bends to make the phasing stubs and threading the spacers on.

The WB6BYU post just came in while I was typing.  I'll play around with the 5/8 and 1/8 idea.

The collinear in the picture is a 4 element, two half waves each side of the center.  They're nicely versatile, you can make a 3 element where the center is a halfwave dipole, two elements both halfwaves, lopsided where you have unequal numbers each side of center, or longer than 4 elements.  This is from an ARRL antenna book, 1960 edition.  The picture is of Hans Michlmayr's antenna in Australia: http://wavelab.homestead.com/index.html (vk6zt)

The other thing about them is that they're in one line, which keeps things simpler since there'll be two of them about 220 feet apart in parallel as an interferometer.  Each antenna goes in a north-south line, and as the earth rotates signals from distant radio sources hit the two antennas with phase angles that are constantly changing, which is what you measure.

Impedances vary depending on the configuration but I've had good luck lately winding baluns on toroids out of old computer power supplies so I can match just about anything to my RG6.  This 4 element should have an impedance around 781 ohms, so a 10.438 impedance or 42:13 turns ratio is what I plan to use.

I know about the reflector wires, but I'm not sure I can get them out of the way of the lawnmower.  Stapling them down and letting them grow in maybe.

The plain piece of wire I'm referring to is each of the halfwave elements, which will be #13 galvanized electric fence wire about 10 feet long.  Maybe a coupling loop around one might work like with a dip meter, but I'll probably just measure with a tape measure.

I haven't seen any of the false dips on this antenna because I haven't gotten that far, but I've used it enough to know there isn't going to be just one.  I seem to get dips on harmonics of the oscillator frequency, that sort of thing.

Radio astronomy types have strange notions of antenna apertures, some probably outdated by ham standards.  Another is measuring signal power as degrees Kelvin and calibrating from noise generators.  From what I can figure there's no direct microvolt or db equivalent because the receiver bandwidth is also involved.  It's a matter of how much energy you can capture.  Systems are calibrated from the antenna to the receiver, with the result that they can read out apparent temperatures of distant objects. I'm new at it and frankly trying to stay away from that area.  All I know is that I can't detect the sun yet, which should be easy.

OK, dangling open wire stubs it is.

Thanks for all the replies.  While it's still on my clipboard here's a picture of one: http://wavelab.homestead.com/east_ant_48.jpg  The article itself was in a 1960 book (paper).

The air insulated phasing stubs are the classic way to do it but there's a picture at http://astro.u-strasbg.fr/~koppen/RJove/antenna.html of using twin lead stuffed into a cylinder to be less obtrusive.  The wavelab picture uses air insulated.

Since it's only for receiving and I don't have to worry about SWR I'll probably just cut it from formulas and put it up, but I thought it should be possible to check the electrical lengths more accurately with the MFJ-269.  I can make the phasing stubs out of coax and look up the velocity factor that way, the elements are just bare wire so I don't have to worry about those.  I can coil up the coax to keep it out of the snow.  I plan to put this about 5 feet above ground (I'm listening to the sky and being low helps cut man-made noise) so I don't want dangling phasing stubs.

I'll check MFJ's site for a newer PDF but the one I've got doesn't cover how to cut a plain piece of wire that's going to be part of something more complicated to resonance.  I suppose I could just stick the end in the coax connector, suspend it about the right height and measure that way.  It also doesn't talk about making 1/4 wave feedline stubs except maybe in the "distance to fault" section.  And of course end feeding a halfwave isn't commonly done.

One thing that bugs me about using it is that there are lots of little false peaks and dips so you have to scan in the right direction, and don't believe every one you see. You have to pretty much know what to expect before you start.

I had modeled this so I made a copy and changed the lengths of the phasing sections about a foot, and sure enough it made very little difference in either the pattern or gain.  Maybe that's a limitation of the "method of moments" modellers, I was using NEC2.  I didn't try setting up a feed for each element and changing the phases on the elements.  Maybe it isn't calculating the effective delay in going though the phasing stubs for some reason. Changing them both by 20% or more should make some difference.

  Alan

I want to build a 4 element collinear (horizontal, for 48.85 MHz, not vertical) from a 1960 ARRL antenna book.  I thought I could use my MFJ-269 to check the lengths of the 1/4 wave phasing stubs and 1/2 wave elements, but from the manual it's not clear just how.

The phasing stubs will be 1/4 wave pieces of zip cord, probably coiled up.  I don't know the velocity factor.  The 1/2 wave elements will be about 10 foot long pieces of plain bare wire.  Measuring the complete antenna isn't going to tell me much because it's a 4 element collinear with 4 half wave straight sections and 2 phasing stubs.  I need to cut the pieces to length before I put them together.  The 48.85 MHz is a locally quiet frequency for doing radio astronomy.  Aperture counts here, which is why I want to use a collinear.

Thanks,

  Alan, ab1jx

Which brings up another question: Aren't the LNBs they use just LNAs?  They don't actually try to downconvert with oscillators running out there in temperature extremes do they?  I assume the downconversion happens indoors and what's outside is just an amplifier.

I know people reload eproms for things like the Kindle Fire to run Linux on them, I thought maybe some people had set some of these up for general purpose use.  I suppose the only incentive would be for stealing TV, and you'd be hard-pressed to pay me to watch most of it.

http://www.mrao.cam.ac.uk/surveys/8C/8C.gif Is just a neat picture of a 38 MHz Yagi (one of 50) used as part of a radiotelescope in the UK in the 80s.  That's a road it's sort of straddling.

About the only things I've got that will receive 1 GHz and up are some old satellite TV boxes I got from the dump.  I don't care anything about watching TV on them, I'd like access to the frequencies they can tune, just for receiving. Getting a pair of them locked onto some frequency as an interferometer would be the ultimate goal.  Actually that might not be so impossible: if I remember right NTSC video is AM modulation so if you use the box to downconvert to TV channel 3 or 4 (common RF modulator output) you could pick signals out of the video passband (about 4.5 MHz wide I think).

But it's in the world of proprietary surface mount chips and unreadable eproms.  Anybody doing anything with turning some of this trash into useful receivers?  You could maybe almost run Linux on some of them instead of sending them to landfills.  Certainly they could be reprogrammed: they're made to accept firmware upgrades.  I've got an RCA/DirecTV DRD420, A Samsung SIR300W, a Dish/Echostar DP301 and a couple others.

There's a site on using some of the newer ones at http://lea.hamradio.si/~s57uuu/astro/sidi1/index.htm but all my stuff is older than that.

  Alan, ab1jx

I've done some frequency surveys (logging activity levels at different frequencies 20 - 70 MHz) for radio astronomy purposes, and most of the time I hear at least some traffic there.  Sometimes not, and sometimes it doesn't sound local.  If you were trying to raise someone there in an emergency you might have about a 50% chance. For non-emergency ragchewing who knows.  I'd keep it and maybe put it on 10 meters, especially if it does SSB. See Ken's Electronics http://www.kenselectronics.com/lists/cbxtal.htm for crystals that will move synthesized CBs to 10 meters with a little retuning.  I've never bought from them, I just had it bookmarked.

 Alan, ab1jx

I put my fat dipole modelling adventures online at http://ab1jx.webs.com/toys/jove/antennas/fat8/index.html.  It doesn't look so bad to me, and I can build it without spending any money.  I just haven't figured out the best way to drill holes in CDs.  14 meters is a full wave, sorry.  NEC2 file's linked on the page.

I just looked again at the BF988 datasheet: surface mount with leads isn't so bad as chip resistors and capacitors.  I've got a small collection of old satellite TV dishes from the dump and taking apart the LNAs often yields some interesting parts, but usually without part numbers. I haven't gotten my new parts into jars yet, I have to go through boxes of bags of parts to find anything.

I've got some J310s, also J309s which I thought looked "hotter" (Idss wise).  I've got one radio which had a Sony 2SK152 that got fried and I found it was almost legendary.  I've got a J309 in there now and it works OK, but it's a portable radio you have to hook an external antenna to for HF. I wish it had been designed around more mundane transistors.

If the MAR6 amps don't work out for some reason I might try something like the popcorn amp on the page you sent a link to but without the tuned circuit on the input.  Maybe rather than a tuned input I can use tunable traps to attenuate problem frequencies if I can find them.  The nearest broadcast station is about 17 miles away on the other side of a mountain.  The Norton amps look interesting too: I just realized they're broadband.

I just finished uploading my collection of spectrographs to http://ab1jx.webs.com/toys/spectra/archives/index.html  The 6th one down has a little bit of the cable TV amp which cuts out below 45 MHz.

I think I've seen what you're calling a fan dipole TV antenna, or maybe not.  They had a pair of elements in front then 6 more elements behind fanned out vertically as a reflector.  Never saw one up close, just on rooftops.  The same technique is also used in multiband scanner antennas.  What I'm also thinking of is the way hams used to cut multiband dipoles out of 4 and 5 conductor flat rotor cable.  Maybe those are called parallel dipoles.

I didn't know the 40673 was gone, it was just a number from memory.  Or that the BF988 was surface mount.  I probably could deal with it, but I use thru-hole when I can.  It's mentioned in the schematic at http://wavelab.homestead.com/HF_VHF_Rx_frontends.jpg, but I guess being smd is why I didn't spot it in the picture at http://wavelab.homestead.com/HF_VHF_module_view.jpg  I have a copy of the PCB artwork but it still didn't dawn on me.

I'd seen the zig-zag lpdas but I thought they were just some fad.  I've got Cebik's PDF on them now.  I could build a lpda out of wire, but we usually get at least one good ice storm most winters where there's about 1 inch of ice on everything.  I am favoring cheap and replaceable (come spring) though.

I hope to stay below 88 MHz and away from the FM broadcast band, but constant signals are just horizontal lines on a spectrograph that you can ignore.  If you take a look at a real spectrograph like http://ftp://www.ips.gov.au/data/Solar/Learmonth%20Observatory/Historical%20Data/Daily%20Spectrographs/02092012spectrograph.gif you'll see bunches of them.  I do see a band of trash from 40-45 MHz on most of the antennas I've tried, which may come from the house.  With a dipole like my fan dipole I can face the end of it at the house so that nulls out a lot, but with the T2FD it's back.  We do have police and fire on low band VHF here still, but it's gradually moving to UHF.  Electric companies and others use low VHF too.  I can see Mount Greylock and Mount Tom about 20 and 30 miles away, and both those (I think) have 6 meter repeaters.  Mount Greylock's 2 meter machine comes in on my HT with the rubber duck unplugged.

My T2FD is about 18 feet long and 7 inches wide.  MMANA-GAL comes with a few hundred antenna model files and I took a T2FD for 7-28 MHz and scaled it down by a factor of 3.  600 ohm output, so I wound an 8:1 balun to feed 75 ohm line.

I don't know about how much gain or what the noise floor is, I just know I'm not hearing anything yet.  That can happen for a few weeks at a time I gather, but I can't even detect anything from the sun for sure.  In the Radio Jove project at http://radiojove.gsfc.nasa.gov/ you put up a pair of dipoles and run a receiver with a pair of J310s in the front end and you should be able to hear/see something fairly often on your chart recorder.  It seems to get a little quieter at night, but that may just be because businesses shut down at the end of the day.  People do use other receivers not unlike what I'm running.

Preamps: I tried a cable TV amp and I could tell immediately that it was dead below 50 MHz.  I thought my antenna connection was intermittent, but it all depended on the frequency I was scanning at the time. I'm just a believer in putting a preamp at the antenna, especially running 200 feet of cheap coax. I used to install TV antennas in fringe areas and we used preamps on maybe 25% of the 300 ohm setups and 90% of the coax ones.

Heatsinking an MMIC noted, also a bias resistor as recommended in the datasheet.

I spent much of the day modeling a fat dipole and thinking about how to build one cheaply.  I've got a 1/4 mile spool of galvanized steel fence wire, also some used network wire I can separate into individual wires.  And a stack of useless "coaster" dead CDs.  If I can melt or drill holes in them near the edge, hang them from the steel wire by 1 hole then put network wires running parallel in 7 other holes it looks good.  I modeled one 14 meters long and it looks pretty good, but not perfect.  8 wires is a significant improvement over 4. I used to have a fat dipole built from old aluminum storm window casing that I used vertically as a scanner antenna and it was pretty amazing.  I'm not sure about wind and ice though.

My question was about powering an MMIC, now we're wandering. I probably won't stay with 30 - 60 MHz because this ties up my IC-7000 and I can't afford another.  Hopefully with the MC13135s I can come up with something else, either several of them on different frequencies or sweeping with 1 since they're VCO based.  If I can make a varactor tuned frontend for it that tracks using the VCO voltage that would be the ultimate.  The really good machines that do this sweep ten times per second, not per minute.  I'm not sure how the time constants on the VCO/varactor control line will work out.  I don't need to use a PLL, so there isn't a lock time to worry about.  Just a range of voltages with trimpots for setting the endpoints should be good enough, especially if I can rig up a marker or two using crystals.  Like a sweep generator in reverse.

I might get steeper skirts with the high and low pass filter combination. As for odd impedances I expect to use 75 ohm, since I got 500 feet of RG6 for $32 at Home Depot.  I have a bunch of RG59 also which is more manageable indoors and has copper conductors.  The MAR6 data sheet says its designed for 50 ohms but hopefully 75 is close enough and is widely used.

I bookmarked the Elsie download page.  I already have a program called Adlab which is similar.

So on front ends, is a 40673 about as good as a BF988?  I wish I could frequency track the remote preamp, but the plans I have use a BF988 as a front end for each MC13135.

Any insights on suitable broadband antennas?  I tried a fan dipole with 17 dipoles (http://ab1jx.webs.com/toys/jove/antennas/fan1/index.html) and now a T2FD.  I might try a really stubby LPDA sitting on the ground pointed straight up.  I don't really know much about radio astronomy, I'm just starting and trying to get antennas up before snow.

I'm sweeping 30 - 60 MHz about 10 times a minute so single frequency or tuned amps are out of the question.  A bandpass filter might be a good thing though.  The choice of 30 - 60 MHz is because that's one of the ranges my IC-7000 will tune without any relay clicks, so I think it's all done with varactors/varicap diodes.  See my page at http://ab1jx.webs.com/toys/spectra/index.html  The picture at the bottom of that page shows a 24 hour 30 - 60 MHz scan using my longwire.  Life was more exciting that way, but most of it is manmade, including the intermod.

I already had a copy of the on6mu preamp page.  What I'm basing some of this on is the wavelab page at http://wavelab.homestead.com/HF_VHF_multi_index.html.  I've got a dozen MC13135 chips on order as well as my MAR6s.  He's using a MAR6 in his preamp between a pair of T2FDs and the coax and another in the signal splitter indoors where he feeds 7 front ends (actually MC13135 receivers).

What I find annoying are the cheap ones that are supposedly temperature controlled are actually power controlled.  There's something like a light dimmer in there: you want to solder something big you need to turn it up first.  I've got a Velleman like that now, I had a cheap red Weller before.  No digital control, no thermostat in the tip clicking away.

They sound like a light dimmer in an HF radio too.

  Alan, ab1jx

Actually this is for amateur radio astronomy use.  I've got a T2FD antenna I built for 30-60 MHz use and the 200 feet of coax is to get away from house RF trash.  It's very quiet, bordering on deaf.  I see a couple of local 6 meter repeaters but that's about it.

I'm sweeping it with my IC-7000 and that shows intermod with the internal preamp(s) turned on.  There's a picture at http://ab1jx.webs.com/toys/jove/antennas/fan1/images/tue.jpg with the white vertical streaks being intermod.  The antenna there was a fan dipole, which seems quieter than the T2FD (I just put up the T2FD today).  Galactic noise would be welcome at this point.

This may border on a dumb question.  I've got some MAR6s ordered, one of which I want to use at the end of 200 Ft of RG6 as a receive preamp to overcome line loss.  All the preamp circuits I've run across for these use an RF choke and bypass capacitor on the output, assuming you're going to feed it direct to a receiver.

Can I bypass that step and connect my output to the coax center wire, then feed power to the coax with an RF choke in the house as well as pulling signal off with a coupling cap?  The coax would become part of the "tank" circuit for the MAR6.  It's probably either a crazy idea or the way everybody does it, I don't know which.

I just don't see jumping through hoops to separate DC and RF as though they were going different places, then turning around and putting them back together again, then separating again in the house.  I want the preamp to be powered over the coax anyway.

Thanks,

 Alan,  ab1jx