Multibanding the W3DZZ

Transcription

1 Multibanding the W3DZZ G3XSD s personal aims: To build a wire antenna for 80/40/30 to complement the existing rebuilt original CobWebb 20/18/15/13/10, that fits the estate ( 70 X70 ) Secondary aims: To modify if possible to allow multiband (5 or 6 bands ( with or without an ATU) to provide an option to others with small estates

2 Multi-banding and myths SWR- low SWR defines how well an antenna radiates - No. Think about a dummy load! No-tune transmitters have 50 ohm o/p because dipoles are the best radiators -No. A double Zepp is 3dB better but has a 126 ohm i/p Z! An ATU tunes the antenna - and a steamroller rolls steam! No. An ATU is an impedence matcher. An ATU is inherently lossy - No. Cheap/small ones are, so are most auto tuners due to the small size of inductors. Proper (usually large and expensive) ones are very low loss More current flows in resonant antennas -not really. Baluns transform according to their ratio -no. But they introduce losses. Best effect is to reduce unbalanced currents on coax feeders. Worst effect is harmonic radiation due to toroidal core saturation ( if used)

3 What informs my antenna implementations Radiation efficiency Losses: ohmic losses, ground losses,radiation resistance Aerial aperture -wire gauge; even balanced antennas have ground losses-so make it as high as possible Spatial size of charge acceleration areas -antennas less than 0.66 of a half wave on the lowest operating frequency will have degraded radiation performance ( size matters) For multi-banding look for SWRs under 3:1 if possible to allow rig based ATUs to keep costs low if cost is no object then neither is SWR it just does not matter. To comply with my secondary aim then SWR <3.0 is the target.

4 The candidate - W3DZZ This does 80/40( no ATU) in its original form First multiband version by OH2EC Improved by OZ1CX who made it a 5 bander

5 The G3XSD design Limitations of the real estate 70ftX70ft House in a stupid place on the plot

6 Variation caused by limitations 100ft max run as a near horizontal vee (80/40) - amusingly the vee angle is 100 degs! 30m inverted vee orientation Max height ( roof height) 23ft (house is a chalet) OZ1CX is 108ft+ * G3XSD antenna will be shorter

7 132FT DIPOLE vs G3XSD/W3DZZ Theoretical expectations Band 132 ft dipole Wavelengths In antenna Feedpoint ¼ waves 80 half Key: *=because of the // 10MHz dipole **= due to L of traps ^= should be reasonably useable ^^=due to resonant traps 100ft G3XSD with traps 125ft Feedpoint ¼ waves 0.947**^ 1^^ 1* 3.765^ ^ L,C,R R R R L L L L R Match on odd number ¼ waves Worst:13m,17m,20m

8 Theoretical conclusions Really not too bad 80m ought to resonate within the band 17m/15m/20m furthest from odd 1/4 wave boundaries, but 80/40/30/13m/10m look promising 41ft 10MHz dipole could be trapped with Sotabeam mini traps at 33 6 to yield better 20m performance Column 2 fractions of ¼ wave Effect of the two dipoles in //? Probably very little. Actual effects of site to be tested pragmatically. Possibly better if antenna length adjusted but given the difficulty of doing this test antenna unchanged. Might be possible to adjust length with capacity hats

9 The proof of the pudding... Five tests for evaluations: Actual global performance Use WSPR over fixed period for consistent conditions using benchmark CobWebb omni dipole of known good performance to evaluate G3XSD version of W3DZZ Measured groundwave field strength Fldigi signal/noise (idling signal) ratio (db) for accurate and consistent automated measurement at a station outside proximity field radius ( typically 5 to 10 miles) thus measuring radiated field. Ensure no sky wave NVIS. At present e.g. midday for 80m, 17 to 10m anytime!

10 Tests (contd) Given decision not to modify antenna, measure swr on each band at resonance AND at nearest digimodes frequencies. Also check if matches can be achieved using internal rig ATUs for bands worse than 1.5:1 using TS-590 ( mid range price), FT-450D (cheap Txcvr), and effectiveness of usage of external ATUs using Palstar AT1KP (expensive),mfj901b ( cheap and portable [small]) Fourthly, WebSDR reports for the 80m dipole because direct remote S-meter readings are available

11 Tests (contd) Given decision not to modify antenna, measure swr on each band at resonance AND at nearest digimodes frequencies. Also check if matches can be achieved using internal rig ATUs for bands worse than 1.5:1 using TS-590 ( mid range price), FT-450D (cheap Txcvr), and effectiveness of usage of external ATUs using Palstar AT1KP (expensive),mfj901b ( cheap and portable [small]) Fourthly, WebSDR reports for the 80m dipole because direct remote S-meter readings are available

12 One last test problems on the feeder? Would be evidenced by RF in the shack due to common mode currents on the outer surface of the coax screen The test: take swr readings then connect an extra 12 ft or so of extra feeder in line: take swr readings again if they are changed you have problems! You are certainly radiating from the feeder and your swr readings are meaningless Let s take a look at the WSPR test results

13 WSPR tests G3XSD/W3DZZ 5W 20% tx periods 20m UT CobWebb 5W 20% tx periods 20m UT Condx the same

14 Groundwave signal strength db Antenna G3XSD/W3DZZ Antenna CobWebb Frequency (MHz) db s/n Frequency db s/n Fldigi in idle mode. Ground wave should decline gradually with frequency increase. G3XSD/W3DZZ produces useful results, except for 13m (not much loss at this end of the cycle!).

15 SWR,resonances & matching G3XSD/ W3DZZ From MFJ-269 (x)from Palstar Ib inband Ob out band swr Z (1.1) 1.9(1.5) L (ib) (1.0) 1.2(1.22) R (1.0) R L(ob) L(ob) L(ob) L(ob) C(ob) ATU Mid-cost cheap cheap band 590 rig 450D rig MFJ Nn or Y Nn or Y Nn or Y 40 Nn or Y Nn or Y Nn or Y 30 Nn Nn Nn 20 Y N Y 17 Y N Y 15 Y Y Y 13 Y* Y Y* 10 Y Y Y The expensive Palstar AT1KP matched all necessary bands as well as the 590 internal and MFJ901 portable ATU, but like the 901 required manual adjustment. The 590 was automatic. The AT1KP was very low loss.

16 WebSDR NVIS results for 80m from Rugely (Staffs) and Weert (NL) G4FPH SDR Cheshire 1842UT 17/04/ W 10 over 9 ( S9+10dB) Dusk [61dB achieved as peak minutes later]

17 SDR Weert NL Nightowls SDR Weert NL in the south near Belgian border -1840UT dusk 17/04/2017 S9+

18 . results of that test for common mode currents Two different lengths of extra coax were inserted using two barrel connectors This gave three sets of swr readings over 80 to 10m All three were the same no problems

19 Conclusions It has been a success by my definitions and aims - 7 bands have good radiation performance, only 13m did not produce strong radiation ( but did have a comparable SWR to the other bands) proves that SWR is no indicator of antenna radiation efficiency Received signal strength is remarkably constant and usable across the bands Variants: W3DZZ 2 bands,oz1cx 5 bands*, G3XSD 7 bands** *G5RV type match **ATU match