VNA measurement of antenna problem

Thread Starter

Janis59

Joined Aug 21, 2017
1,415
Have to build marine antenna, thus for 159.5 MHz the vertical dipole is just what is needed (sometimes called bazooka with stright-lined contraweight). Took the H-2000 very high quality cable (din 2.4 mm, D insul 7 mm, Dout 10.2 mm, 50 Ohm) and get clean of sock the upper 45 cm. Then down I tried with soch but it looks ugly, tried copper pipe but then the aoverall plastic pipe for beauty becomes too massive, thus I found the idea to solder 12 small wires from end of cable sock downward 45 cm. Now construction is suitable for din 16 mm santech whitepipe. Thus antenna mechanics is brilliant yet the VNA shown the valley of tears. Whole the VSWR picture is full of hedgehog teethes every 1 MHz the one after one. And average SWR about 6...7. Except the one abyss with SWR is 1.5 at about 110 MHz. Thus, logically, I suspected the shortening factor effect so bite off 5 mm both ends, then again, then 1 cm, then 1 inch until had a 25 cm at the hands. The blossoming 1.5 valley still happened at 110 MHz and nothing was changed. So, what a heck? Of course, such antenna theoretically have about 60 Ohms, thus the circulating power between antenna and cable is inevitable, but not very strong. Maybe the cable length (about 15 meters) is responisible, but I cannot cut it shorter to avoid the rain and salt coming-in. So - what are Your ideas what I should do?
 

Ya’akov

Joined Jan 27, 2019
6,264
What is the material of the pipe? Is it PVC?

The marine band antennas I know of are base loaded ½λ whips in fiberglass radomes.
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,415
Yaakov, its white sort of Polypropylene. I tried to take the graphs without the pipe as well, the graphs are unchangeing. So my decision is that inner to and fro reflections makes the strong `noise` masking the signal. Theoretically, there MUST exist ONE frequency on what everything is okay, seems that is the valley. But why it not changes with the change of resonator lengths?? In attachment VSWR graph as function of frequency. PS: does the cable have 50 Ohms - I checked that, has.
 

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Ya’akov

Joined Jan 27, 2019
6,264
Do I understand correctly your 16 wires are currently parallel to and against the cable jacket?

Is it possible to test with the wires away from the cable?
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,415
RE: ""I know of are base loaded ½λ whips"". Generally its true that marine antennas has distance limited by power/antenna G factor and sensitivity/noise about 200-400 km, whilst the Earth curvature limits the waves at 20 km or in best case 40 km. Thus, none cares about G-factor etc. Thus the whip is good enough, but whip and bazooka differs so tiny, mechanically. About bazookas ground part: dipole means two identical vibrators. The upper one or hot one is just the coaxial middle wire with insulation with taken off braiding. The lower one or "earth counterweight" is bunch of multiwire stubs hanging down from most upper point of braiding, let them as better as can are shielding off the braiding created field. Plus that, to avoid the RF current travel down by the mast (aluminium, grounded), the coil of 50 mm inner with 9 turns are used just beyound the end of counterweight. Later I tried three ferrite rings instead and also surplus to coil. And tried the coilless and ringless - situation is unchanged. Example of bazooka is here, clearly see the lower part=coil, middle part=counterweight and upper section is middle (hot) conductor. https://www.passion-radio.com/2791-thickbox_default/bazooka-pro.jpg
And here is bits of theory from AARL handbook about it http://www.iw5edi.com/ham-radio/3777/the-vertical-bazooka-antenna
 
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Thread Starter

Janis59

Joined Aug 21, 2017
1,415
OK, shall try to set those wires 90 degrees. Have no doubt then all will work, But shall see by the experiment.

P.S after 5 minutes: it worked, the valley jumped to right place and gives VSWR about 1.2. But, geometrically counterweight cannot take the wider place on the mast not harming the sail. It cant be exploited this way. But at least I can now play with knitting the counterweigt around the coax with less gaps and more "insulating" it`s field. Yet today I must end to play, its late, lets go forward tomorrow. Thanks for Your idea gave me understanding whats wrong.
 
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Ya’akov

Joined Jan 27, 2019
6,264
RE: ""I know of are base loaded ½λ whips"". Generally its true that marine antennas has distance limited by power/antenna G factor and sensitivity/noise about 200-400 km, whilst the Earth curvature limits the waves at 20 km or in best case 40 km. Thus, none cares about G-factor etc. Thus the whip is good enough, but whip and bazooka differs so tiny, mechanically. About bazookas ground part: dipole means two identical vibrators. The upper one or hot one is just the coaxial middle wire with insulation with taken off braiding. The lower one or "earth counterweight" is bunch of multiwire stubs hanging down from most upper point of braiding, let them as better as can are shielding off the braiding created field. Plus that, to avoid the RF current travel down by the mast (aluminium, grounded), the coil of 50 mm inner with 9 turns are used just beyound the end of counterweight. Later I tried three ferrite rings instead and also surplus to coil. And tried the coilless and ringless - situation is unchanged. Example of bazooka is here, clearly see the lower part=coil, middle part=counterweight and upper section is middle (hot) conductor. https://www.passion-radio.com/2791-thickbox_default/bazooka-pro.jpg
And here is bits of theory from AARL handbook about it http://www.iw5edi.com/ham-radio/3777/the-vertical-bazooka-antenna
I’ve built such antennas specifically for portable use on 2m and 70cm. The worked reasonably well, but I usually preferred end fed ½λ whips because, although they were more complicated, they could be mounted directly on the portable radio, or on a mag mount BNC base and the flexibility was handy.

You should be able to make a working version for the marine band. It seems like something odd is going on.
 

Halfpint786

Joined Feb 19, 2018
40
When you see many SWR dips at regular intervals like that, it is almost always caused by the coax acting as part of the antenna. If the outer shield is acting as an antenna at one frequency, it will also act as an antenna on harmonics of that frequency, hence the "teeth". This is a good indication that a better common-mode choke is needed at the feed point.

The trouble with bazooka "baluns" is that the bazooka frequency will never match the counterpoise frequency because the velocity factor depends on the insulation. Pulling the shield back over itself (one side near insulation, the other not) causes the VF of the bazooka and the VF of the counterpoise element (same wire, different sides) to be different. Does it work in a pinch? Sure. But it is not ideal and not easy getting right.

When you tried coiling up the coax, did you base the number of turns off of anything? There is a sweet spot with the number of turns in a wound coax balun (and it depends on coax and frequency). In other words, more turns isn't always better. The coil inductance and the inter-winding capacitance form a parallel resonance that represent an RF open circuit at that point. One might think that any inductance is better than nothing and just wind some random length up, but if the coax shield going to the radio (the the part that is acting like an unwanted antenna) has, for example, a capacitive reactance, adding a random coil in the coax could work in series with that capacitance and the reactive portions cancel out. That can result in a coiled up coax making things worse. Make sure you have the coil dimensions right. If you try to move the counterpoise radials out and away from the coax like a ground plane antenna, you should move the coax "coil" up to the feed point.

Back to the bazooka. The diameter of a coaxial filter (bazooka) has an impact on choke impedance. Coaxial cavities and hairpin inductors alike, the impedance looking into it is a function of the characteristic impedance of the line used to construct it and the wavelength. At resonance, the reactance of a 1/4 wave shorted line is theoretically infinite, but in reality, it is nowhere near that. It can be improved by making the bazooka characteristic impedance higher. This is why, IMO, manufacturers of bazooka antennas (and duplexers) use the largest diameter practical. Back to the plastic pipe issue... can the bazooka element be a copper pipe on the outside of the plastic pipe enclosing the rest???
 

LowQCab

Joined Nov 6, 2012
2,329
Pictures, Pictures, Pictures ..................

A verbal explanation is not adequate when talking about Antennas.
Especially when there is also a Language-Barrier getting in the way.

This Antenna design is almost bullet-proof, and dead simple,
when implemented exactly as shown in the link.

Much higher performance could be had with a "Bi-Cone" Antenna design.
Much less Atmospheric-Noise, plus much higher Gain, plus very broad Bandwidth.
.
.
.
Bi-Cone Antenna Dimensions .PNG
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,415
Yaakov: Constructed the copper foil counterweight and it worked well.
Halfpint: Yes, thatswhy I tried the ferrite rings with and without a coil. No effect. However true, the coil I made by idea bigger is better. Now, when foil is just about working, I have to play with the number of turns.
Halfpint: Probably I shall wind the larger diameter indeed, I have high quality Belden H-2000 cable not permitting small bending radius. Copperpipe for this cable have to be 15 mm and then the 20 mm plastic pipe is too small. If would exist the 13 mm copperpipe, I would go this pipe-way for sure.

LowQCab: hmmm, never heard. Shall read about, as the bandwidth of mine is very poor indeed. Gain, as I noted before, for marine VHF is nothing worth to pay - distance limited by Earth curvature 20-40 km, distance limited by gain and sensitivity - near to 1000 km. Wins shortest! But according Your drawing, there is large mechanical problem - mast is something at 10+++ meters high with winds in extreme near 200-300 km/h where only point of mount is tiny platform 10x20 cm and one 8 mm screw; below stands the whole time shivering sails afraid to catch-in. Above stands thunder arrestor what must be in near field distance at maximum 20 cm afar where it enormous disturbs. That all makes a thick knot of problems with Bi-Cone. But thanks for the care!

P.S. I dont like a perfectly, but I made the antenna is rotatable by a rope. When rope is pulled, antenna is up. When rope is released, antenna falls so the arrestor is highest element for case of stormy weather. Anyway, while the thunderclouds, there are no radio conversation I know from my experience with 34 ft sailship what was lost in the storm epicenter. I ride with solid circle few miles afar of it so escaped the "washing machine" and was about to declare the port-captain emergency with that ship (his last words was, I cant speak, the flashes happen each few seconds both sides of borts, but after two hours radio revived and he told that was blown out into open sea from the Gulf and radio under thundercloud was silent. After that I made the arrestor on my mast and throwable emergency grounding plates. Sometimes may happen the prognosis says wonderful weather and sun, but in 5 minutes sea becomes monstrous for not long.
 

Halfpint786

Joined Feb 19, 2018
40
Yaakov: Constructed the copper foil counterweight and it worked well.
Halfpint: Yes, thatswhy I tried the ferrite rings with and without a coil. No effect. However true, the coil I made by idea bigger is better. Now, when foil is just about working, I have to play with the number of turns.
Halfpint: Probably I shall wind the larger diameter indeed, I have high quality Belden H-2000 cable not permitting small bending radius. Copperpipe for this cable have to be 15 mm and then the 20 mm plastic pipe is too small. If would exist the 13 mm copperpipe, I would go this pipe-way for sure.

LowQCab: hmmm, never heard. Shall read about, as the bandwidth of mine is very poor indeed. Gain, as I noted before, for marine VHF is nothing worth to pay - distance limited by Earth curvature 20-40 km, distance limited by gain and sensitivity - near to 1000 km. Wins shortest! But according Your drawing, there is large mechanical problem - mast is something at 10+++ meters high with winds in extreme near 200-300 km/h where only point of mount is tiny platform 10x20 cm and one 8 mm screw; below stands the whole time shivering sails afraid to catch-in. Above stands thunder arrestor what must be in near field distance at maximum 20 cm afar where it enormous disturbs. That all makes a thick knot of problems with Bi-Cone. But thanks for the care!

P.S. I dont like a perfectly, but I made the antenna is rotatable by a rope. When rope is pulled, antenna is up. When rope is released, antenna falls so the arrestor is highest element for case of stormy weather. Anyway, while the thunderclouds, there are no radio conversation I know from my experience with 34 ft sailship what was lost in the storm epicenter. I ride with solid circle few miles afar of it so escaped the "washing machine" and was about to declare the port-captain emergency with that ship (his last words was, I cant speak, the flashes happen each few seconds both sides of borts, but after two hours radio revived and he told that was blown out into open sea from the Gulf and radio under thundercloud was silent. After that I made the arrestor on my mast and throwable emergency grounding plates. Sometimes may happen the prognosis says wonderful weather and sun, but in 5 minutes sea becomes monstrous for not long.
What about copper foil tape for the counterpoise on the outside of the plastic?
https://www.amazon.com/LOVIMAG-Cond...659025060&sprefix=copper+foil+,aps,215&sr=8-6
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,415
So, todayin the end of working day had a bit time to wrap counterpoise in 0.15 mm copper film, longitudinally to exclude any turns, soldered cable sock, fixed by scotch transparent tape and mechanically its is rather beauty. However electrically.... made the choke balun with 10 cm diameter and 6 turns - SWR about 6. Add one ferrite bed, SWR about 5. Straightened coil and taped longitudinally the cage made of 4 antenna I-cores gaplessly. SWR fell on 2. Added to it one ferrite bed, SWR 3. Add two ferrite beds SWR 2.5. Tok them off, 2.0 However for 25 Watts seems over the 1.5 would be big mistake to use anything. So, only idea that antenna core was HH150 thus may be the 160 MHz is bit much for it, but ferrite beds was from computer displaycords, thus probably the same cause?
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,415
RE:""foil tape for the counterpoise on the outside of the plastic?"" Inside the plastic pipe but outside the coax cable.
OK, I killed my sloth and made o picture for antenna state today. One in composed state, another with nude state. IMG_20220803_105208.jpgIMG_20220803_105225.jpg
 
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Thread Starter

Janis59

Joined Aug 21, 2017
1,415
Update: have to max 5 minutes every day to solve this antenna question but at least have find following:

1) when radiator is insulated with thin in comparison with own size insulation, like pipe over antenna whip, the V(F) change is extremely small and many cases may be neglected at all. When insulatrion is thick, as central wire of coaxial, it gives impact to V(F) if Earth (or outer sock) is near - none a case here. But diameter of radiating wire have an impact, thus my case 159.5 MHz for 11 mm lower radiator must have, according the calculation, 95 cm, whilst the upper 2 mm thick must have the centimeter shorter, 94 cm. Testify and see... ANTIresonance at 130 MHz and no more resonances except very weak around 84 MHz. Totally wrong. Calibrated measurer, 50 Ohms test impedance shown 50.03 and another made 20 years ago 50.05 Ohms. Count the apparatus shows correct result, at the "resonance" it R=tens of kiloohms... sure proof for serial antiresonance, or half-wave resonance. Thus, 84 MHz may really be the true resonance but then WHY, the velocity factor of free air 0.5??? Never!

2) Okay, think probably problem is caused by long cable between antenna and measurer. Made the another antenna with some three turns coil over 32 mm air core and after centimeter the plug PL252 and measurer. Reaction the same - within SWR=3 are available some two or three ranges where it is possible to adjust something, but not less that 3. Identically antenna with full length cable having no coil but 10 pieces 10-400 MHz validated ferrite cores with no gap over the cable, on 10.5 mm outer stays 12 mm inner thus gaps are rather well. Say, influence from cores was significant, to differ those ferrite rings I took from PC display cables, those have antiresonance at 100 MHz thus completely non-effective.

3) Thought, okay, probably ought experiment with identical diameters of both radiators, lower and upper, thus soldered upper to 12 mm copper santech pipe. Lower 11 mm. Result - best SWR 3 and no a yotta less. WHY?? Repated experiment on the full length cable with ferrite rings, the same, as best SWR was 3.8 as now the best is 3.5, knockout....
What there is happening?? PS: nearest metallic constructions are at least 4 meters away, except the mains grid and internet cables staying 2 meters afar. The H over physical Earth is about 10 meters (3rd floor).

4) Touching the hand to cable near RF stopper (coil or rings), coil makes reaction about 30% of measured parameter, while ferrite rings version have impact less than few %. Thus, seems, cant blame the parasythics in the cable as well.
5) As I told, the cable came in factory coils, trademark H2000 what is very expensive most ever seen quality ultra-low loss 10.5 mm cable of 50 Ohms, with 3 layers of braiding (copper sock plus 2x copper tape) and massive 8 mm teflon insulator, inner wire 2 mm, mono. Testing the cable, VNA shows no signs of fault.

Indeed cant realize WHAT I do wrong, Do You see some issues to begin with??
RE: Halfpint786
RE: Ya’akov

Pictures? OK, here is short-cable version very the middle, where upper and lower radiator divides and next picture the same "short cable" version the place where cable is coming out from the radiator.Clipboard01bbb.png
 

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Halfpint786

Joined Feb 19, 2018
40
You should not be winding coax coils that tightly (most recent picture). You are greatly exceeding the manufacturer recommended bend radius. When you do that, the center conductor will begin to migrate through the insulation and affect the impedance of the cable. It also looks like the jacket is cracked in that last picture and water will get in and ruin the coax. If that cable jacket is broke there, seal it up.

I've never seen an analyzer like that, and being you are working with VHF, I am curious what you are using as a load to calibrate it. What kind of analyzer is that and what cal load are you using?

What mix ferrite are you using?
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,415
1) Bent radius was 32 mm for form-core, and I checked twice cutting the cable after bend that braiding is not damaged. But migration - hmmm, had no idea to check it, however hard solid teflon is not prone about, but thank You for counsel, shall keep it larger next time.

2) Analyzer is most advanced sub-twin of popular N1201 device with extended lower frequency part and extended high frequency part, namely PS200. Had once compared results with professional device of Rohde&Schwartz and hot about +/-1% the same results. https://www.amazon.co.uk/Vector-Antenna-Analyzer-34-5M‑2-7G-Network/dp/B092RB3TWK

3) The calibration of device is far more stable than in my NanoVNA-v2.0 thus years long the recalibration is not demanded. But actually I used 50 Ohm bean from NanoVNA complectation and other was 50 Ohm standard BNC end-load from old soviet military (unknown what) measurement apparatus, but inscription on it made very clear what it is and for what sake made. Thus, I have no doubt about measurer accuracy, BUT it shows 250 active Ohms at the resonance frequency!!! OK I may understand such if there would stand any folded structure like folded dipole with its 300 Ohms, but here is nothing like that except the lower radiator, but it`s central part CANNOT radiate anything as it is well ecraneted (shielded) by radiating part (copper folie). However folie I was too lazy to solder longitudinally, but wrapping is about 1.5 turns by width, so I have no doubt about shielding completeness.

Seems very satan had drilled in to the cabling in order to test my patience and make me mad.

4) Ferrite was USA-made with articule RI-RT20-12-10 https://www.tme.com/Document/bc58ece56efcb4bf75f4c5dd4118fadf/RTxx.pdf with material code K5B, A5, A6 (unspecified which exactly, probable the mixture between these three). As the impedance at 160 MHz for one ring is, by datasheet graph, about 75 Ohm, then all the ferrites composition on cable ought to give about 750 Ohm for currents rejection. As that is more than minimum 200 Ohm reccomended, I cant blame the rings now.
 
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Thread Starter

Janis59

Joined Aug 21, 2017
1,415
UPDATE:
Made annihilated the dipole project and erected the 5/8 mono pole instead.

Base: hand-sized porcelain insulator into metal keeper into which is fixed 12 mm copper pipe for 1/3 length, in that soldered 8 mm copper pipe for 1/3 length and it that is soldered last 1/3 length with yellow metal 5 mm pipe. The cable central wire is soldered inside the copper 12 mm base, and braiding to that screw keeping the insulator to the mast. Mast I used simulated, 4 meters long steel-pipe of 80 mm diameter just for the experiment.

First surprize: for 122 cm mono pole had the first resonance frequency about 230 MHz (???!!!). Tried to push over the antenna top one 12 mm copper pipe to lengthen antenna up to 170 cm and it resonated at 70 MHz (???!!!). Shortened that pipe until reached my wished 159 MHz and that lead to 122 cm (!!!???). I think its a proof that soldering copper with brass, the upper pipe somehow not working, probably due the thermoelectric contact-voltage or who knows why. Simply copper dont friending with the brass. Or simply because it is significantly thinner than pipe in the antenna base (however gradually diminishing the diameter is frequently used technique in antennas).

Second surprise: antenna "earth" is hand sensitive - the sign that standard 8 Ohm earthening conture is too weak. Or the steel is totally wrong. Anyway, it acts sooner than "bad" contra-weight with defective length not like good earth. So, who knows how the real mast of aluminum grounded into seawater will act. Anyway, seems I must change the antenna resonator to the one-piece pipe what means too much heavy and may act bad in the strong winds. And I must simulate the real mast much better, preferable by aluminum pipe. My workmate said the wrong is that I am trying to use the "absolute earth" making antenna un-symmetric, so I ought to use contra-weigths with adjusted length. But its prohibited, because contraweights then will become tangled up with the sails textile. Nothing may hang down from the mast top.
 
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