HFU HF Underground
Technical Topics => Equipment => Topic started by: ultravista on January 09, 2019, 1802 UTC
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I have been using an active mini-whip antenna (PA0RDT) at approximately 20 feet on a non-conducting mast. From my QTH, the mini-whip has been exceptional for RX.
Over the weekend, I ran a wire around the house under the eaves, approximately 200ft of wire @ 10 feet or more off the ground. As a loop, the coax braid connects to one side and the conductor to the other. It is two conductor alarm wire and I am only using one in the pair. No balun, just a simple PL-259 connector connecting the conductor and braid to the wire.
The mini-whip wholly outperforms the loop antenna. Strong signals present on the mini-whip practically disappear with the loop. On a scale of 1-10 for RX, the loop is a 1 or 2 compared to the mini-whip. Using the loop is like not having an antenna at all.
As a novice, I thought for sure, the long wire around the house would be at least as good if not better due to the electrical length. Not at all.
Can someone help me understand why the mini-whip @ 20 feet is outperforming the 200 foot loop @ 10 feet?
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Hmmm. This is odd as loops are normally very decent antennas, considering the lengths of wire involved. Mine performed very well and was almost identical in layout to yours, fed with rg6 with ferrites at each end, but must admit I don't have a active ant like yours to compare. I'd make sure there's no shorts or bad coax or such forth.
I'd use both wires in the pair as a single conductor, but it'd be fun to see how the loop fared with two turns instead of just one. With two turns you'd likely be better off with a balun, I think most loops have around 1k z at their full wavelength frequency.
One thing to consider is the feed to the active, if you don't have ferrites just under the active on the coax line, the coax line is acting as an antenna feeding the active as the active is a dipole in such a case. Ferrites at the feed point of the active will disconnect the outer shield of the feedline from the active - you may have employed a nonconductive mast but the feedline's acting as one anyway. Also if you use wire rather than via the coax line to power to the active, those wires will act as part of the active antenna dipole. They can be decoupled from the antenna by ferrites also.
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This is interesting thanks -- I have just obtained a PA0RDT and look forward to trying it -- I know height is apparently a key factor with the mini-whip.
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I'd use both wires in the pair as a single conductor, but it'd be fun to see how the loop fared with two turns instead of just one.
I'll give the 'both pairs as a single conductor' a shot this weekend, in addition to a different connection from the coax to the antenna wire. The coax is new but my homebrew connector is about 15 years old. Heck, I should be able to use a clip to connect the conductor and ground to the coax to see what happens.
Not sure about the double run loop. Considering the second run of wire would be running parallel to the first run, wouldn't there be a coupling problem? It would be twice as long electrically but it will share the same stand-offs attached to the wood trip.
The mini whip power supply is in the shack (the dining room table ..) next to the radio and laptop. I have tried ferrites but it didn't seem to help, or at a minimum, had no material effect on the signals visible in the SDR waterfall.
The mast is 10 feet of galvanized steel fence post with a 10 foot section of PVC clamped to it (wooden dowel inside for strength). The active antenna sits at the top of the PVC.
What amazes me is that this little antenna in Las Vegas, NV is capable of receiving NDBs in Montana, Canada, and Oregon in addition to XSG, HLG, HLO, HLF, and HLW Coastal Radio markers when the conditions are favorable. Not bad if you ask me, but as a novice, perhaps something everyone can do.
The active antenna is great, awesome for me in fact, but I would like a working alternative to compare results. I was hoping the loop would be effective; sadly, it is not.
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"Not sure about the double run loop. Considering the second run of wire would be running parallel to the first run, wouldn't there be a coupling problem? It would be twice as long electrically but it will share the same stand-offs attached to the wood trip."
Give it a shot and see how it plays.
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Your results are rather curious. I'd suspect either a faulty component/cable, or perhaps a wiring issue? I'd expect a fairly decent signal from such an antenna, although the proximity to the house means it could also pick up a lot of RFI. I assume the eaves are not metallic?
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the proximity to the house means it could also pick up a lot of RFI. I assume the eaves are not metallic?
You may be picking up noise (EMI/RFI) from the house that has raised the noise floor enough to cover weaker signals.
If the eaves are metal covered they could be acting as a shield.
That said, make sure your connections are properly made and the coax and PL-259 are not shorted.
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You have a surprisingly bad result with this loop antenna. I think the reasons for this are two:
- mismatch of the coaxial cable to the loop
- shielding effect of the building structure (it receives only the loop section from the direction of the incoming radio wave, i.e. you have practically the vile Long Wire).
My Sky Loop is 160m long and runs on trees at a height of 6 to 12m above the ground in a green area surrounded by tall buildings.
The signal is better, less noise than from Long Wire about 40m as well as 2x38m dipole V in the horizontal. I liquidated them after taking the Sky Loop.
It is powered by an RG6 coaxial cable through a 9: 1 baloon.
Works satisfactorily from LW to the end of HF. Comparing it with the Mini Wip of the European KiR SDR receivers often goes out better, but the signal strength can not be evaluated objectively because of other propagation for those locations.
Good luck in tinkering!
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The eaves are wooden, the wire stands off the wood by 1 inch (via ceramic insulator), and runs around the perimeter of the house.
There are some spots where the noise floor is higher while others it is lower.
Signals like WWV @ 5000 khz are strong with the active mini whip and nearly imperceivable with the loop. To me, it is very odd. I didn't expect such signal loss. It is almost like I do not have an antenna attached in HF.
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Signals like WWV @ 5000 khz are strong with the active mini whip and nearly imperceivable with the loop. To me, it is very odd. I didn't expect such signal loss. It is almost like I do not have an antenna attached in HF.
That strongly suggests a wiring problem, either a short or an open.
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Are you absolutely certain that the coax and PL-259 are OK ?
Make sure the PL-259 is not shorted. If it got too hot when it was soldered to the coax
there could be damage in form of a short circuit.
Disconnect one side of the coax where it attaches to the loop wires and read the PL-259
center pin to shell with an ohmmeter... should read open, no resistance.
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I bought a Palomar Engineers MLB-2 Long Wire antenna matcher (9:1) balun (100KHz-30 MHz). I'll use this once it arrives.
The coax is good, brand new in fact, but I'll try another one to test it.
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Do you have a dmm to do some checking with?
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Josh, yes, I have a digital multimeter.
I noticed that on 80 and 60 meters, the loop is comparable to the active mini-whip for amateur stations in California, Arizona, and Northern Nevada - I am in Southern Nevada. So 'local' stations are pretty much the same but elsewhere across the bands, the mini-whip outperforms the loop 10 out of 10 times.
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I'm guessing the active is vertically polarised, and the loop is going to be a nvis antenna more or less. Verticals should almost always be the dx king in the antenna world due the lower angle of radiation.
Something I noted a few times before I turned the loop into an ocfd (obsessive compulsive fed dipole) was a station some 40 miles from me on 40m was clearly heard, but he couldn't hear me as he was vertically polarised and I was on the loop - more or less nvis. I suspect this was ground wave reception on my part, no wonder he never heard me as his antenna could hardly respond to signals coming from the zenith.
http://www.aresok.org/pdf/NVIS_W8CX.pdf
https://region6armymars.org/downloads/NVIS-Antenna-Theory-and-Design.pdf
http://www.n2ckh.com/AAR2EY_NVIS_ANTENNA.pdf
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Josh makes some good points about NVIS - its a fascinating propagation method. A lot of pirate radio reception in the US is via NVIS propagation. The military makes extensive use of it for tactical purposes.
In an ideal world, one would have a vertical antenna for send-it-to-the-horizon purposes and another antenna for NVIS (on HF). Also a dipole. And a beam..