# Vertical Coaxial Dipole question

#### Janis59

Joined Aug 21, 2017
1,356
Came the urgency to arrannge a certain yacht marine radio DIY antenna.
Read the most frequently are used vertical dipole made of stripped cable, let the quarterwave braiding along the coaxial downward forms the one branch and central wire upward makes another quarterwave branch. I like such simplicity, but my all senses signalizing at least a two provocative mistakes must be here.
1) In counseling video www.youtube.com/watch?v=SdEglNHyHB4 is shown that downgoing braiding comes ALONG (near) the upgoing cable. For my imho then upgoing current will compensate the downgoing what is very much wrong. The braiding must be weared fully OVER the incoming cable like women socks when taking off, let the air sees only radiated current but inner is shielded. Am I right, or am I wrong?
2) In hundreds of websites are told - do this dipole and just plug the cable in radio. I feel it is even the more terrible mistake. The plug is assymetric, coaxial is asymetric, but dipole is symmetric. Thus to avoid the radiating from the whole cable braiding being probably 12 meters or even longer, it ought to include the balun. And, to my experience, nothing is so simplistic and effective as just some 5 turns air-core wound the same coaxial cable somewhere rather near to the entering the antenna, I mean dipole lower end. Diameter of this coil is not so much important. Am I right, or that marine antennas ares some sort of grand exclusion what demanding no coil (balun)?
3) those idea about bending the ends of wires around the tubulus ends sounds spooky too for my much suffered ears. I am 3/4 sure those bendd length at least particularly comes in count of resonance. Why not use the fishing thread instead?

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#### Delta Prime

Joined Nov 15, 2019
1,189
Your on a yacht! Put your camera down and grab a watt meter measure your VSWR. That's where I would start.

#### Janis59

Joined Aug 21, 2017
1,356
Not much true. If mast is 11 meter tall it is a kind of monkey job to fix antenna there up and very sorry if need to do it twice. So, it would be catastrophic to find then, that coil WAS needed and braiding MUST go concentrically as the 10 mm cable costs about 80 USD. I must know it beforehand for sure to make any cut by knife at hands. And even if to buy the longest version to throw out only some 2 meter of coil weight if coil is unneeded, even then the all measurements nearer the Earth will give a rather sure deviated results. Antenna or correct construction must stay in the correct height and then the measurer makes the good job.

More over, I am going to buy this waterwonder in neighborous Sweden without radio, so taking a radio and cable in bag because crossing the sea of 200 km without radio is labelled a boating crime, thus, as less I have electromechanical jobs in foreign country with minimum instruments at bag and only mains is 12 Volts battery, as better it would be for me.

My intention was to realize, do You all are in the same mind as me - about coil and braiding, or the video-roll author is right.

#### Janis59

Joined Aug 21, 2017
1,356
By the way. There are actually TWO good methods to adjust the antennas. One is cross-scale VSWR meter where one may see only result without of clear understanding what is what

Completely other method is VNA where one may no need to have an alive transmitter at all. Such budget scale on port VNA is SA1201 what is just modern age wonder. I love it. But it works when knife-cut is done You see where are parallel resonances, where are serial, where their multiplicatives, what are Q factor, what are L and C and X and R and Z etc etc. So the "blindness" is order of magnitude less painful. It works 137-2700 MHz.

Next choice is my Nano-VNA, yet I have a small display unit and may advice any to buy sure the largest display unit. It is two-port machine what made me sit speachless. Most of my capacitors was proven to be good coils, and most of my coils was proven to be good capacitors. More over, even those capacitors (SMD) what seem to be GHz range specified, gave a just resistive-inductive character at 3...100 MHz and only near the 1 GHz became a good capacitors. I may understand if contrary, but this way... oh my.... Anyway now I more valuate the labels of type "use a capacitor part number xxx from producer yyy". Nano_VNA works well between 50 kHz and 900 MHz.

#### Janis59

Joined Aug 21, 2017
1,356
Just foun an order of magnitude more correct article https://vk2zoi.com/articles/half-wave-flower-pot/

For my understanding that is the same fish ony sack with another label. So, I was right that coil is mandatory. But it uses the braided lower part reciprocically. Thus I guess probably I tracted a videoroll advertized antenna wrong, it isnt the full dipole but the half-dipole with quarterwave matching stub!!!
So, that was plainly analfabetic construction and last is absolutely exact. Of course the dimensions must be slightly corrected according to new wavelength. And sorry I have PE-F coaxial instead of demanded PE-Solid. I feel the problems with my tail-bone, but is too late. Of course the other V(f) must be taken in account as well.

So... I count my action plan ready. Thanks everyone for response those who was able to.

#### SamR

Joined Mar 19, 2019
3,609
Congrats on the new boat and I agree, never go offshore without marine VHF. While I do applaud you for trying to homebrew an antenna, matching it to the radio will be difficult. Especially since no marine VHF I've ever owned or used had an integral antenna tuner like my Ham rig does. To save a lot of headaches and get a 3dB gain over the dipole I would suggest: https://www.westmarine.com/buy/shak...3db-vhf-sailboat-antenna--6966089?recordNum=6

#### Janis59

Joined Aug 21, 2017
1,356
Thanks for idea, probably I shall use it one day, but not so soon.

For my calc my tuner 0,4 microvolts at SNR 1:1 and antenna G=3 dB (simple dipole) gives poor but understandable conversation in plenty hundreds of km. However the Earth diameter makes calling impossible already at 2 pieces*3,54*sqrt(H) where H=10,5 so the 60-70 km is most far ever imaginable. Therefore there is no need for good G-factor, completely no need if I haven't higher height.

And about antennas, it will be at least dozen made by me in the past in my 61 year age, different calulated and trimmed by me, so I am not first time with cigar on the hey roof however I am far off from the grand expert. My first antennas for aircraft modells I produced so far as at 1967 or 1968 when everything binded to radiotransmitters was top military secret in ussr. But at least all three instruments I mentioned bit above are mine private, thus the 99% of RF measurements are just to pull it out of the stack question. Even my oscillo are working well up to 1 GHz and with bit form errors up to 2,5 GHz. As the State leading University is too poor to buy good instruments for own electronics designer buereau, so I am trying always to have aprivate and very good tools, however some skyscaping tools like Rhode&Schw are out of even our dreams.
But yes, probably it isnt a bad idea to take with me in the journey the crosscale WSVR meter even if antenna will be adjusted on my house roof. To burn the output transistor is rather troublemaker.

#### Janis59

Joined Aug 21, 2017
1,356
RE: SamR:

 ""We have detected that you are visiting us from a country with Global Data Protection restrictions (GDPR). Due to requirements placed upon West Marine as a result of Global Data Protection Regulation, we are not permitting internet traffic to our website from certain countries in order to avoid any violations. No tracking or performance measurement cookies were served with this page.""
[TD]""Page Blocked"" [/TD]

Seems I should concentrate to open that State Most Secret Place by means of Onion. Questions before opening, first raw: which State and with which IP You want to emulate Yourself. Second question - via what servers You want to make hops and how much. Tor is really strong thingy but exists even stronger things for fools like guys being afraid to show the webstore advertisements. They just spit straight in the soul. We are NATO full rights memeberstate somehow and have visa-free travel regime with USA. What to hide from us?

#### Janis59

Joined Aug 21, 2017
1,356
RE: SamR
Aha! That is quarterwave shortened CLC for 56 USD and plud for another 56 (!)
I am buying the PL259 for 3 USD and materials for diy dipole (having bit better G) costs under 1 \$ except the cable, but I have it much at my shelves.

By the way, I cannot to see any good into put a plug at the mast-top. Any plug is object to corrosion (even if hermetized in rubber), RF losses and reflections etc etc. Therefore VK2zoi design I like much better (for a while) -

And... I catch a glimpse that Amazon antenna have not done anything to avoid the RF radiation from the coaxial, along the shielding. Maybe I am wrong and there somewhere is well hidden the ferrite core, but cant see it, so if it isnt, the construction is not good.

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#### Janis59

Joined Aug 21, 2017
1,356
Made a bit calculus to prove the said that antenna gain is not playing ANY role in UHF.
So, My MRF75D model have 0,30 microvolts sensitivity at SN ratio 12 dB. At 50 Ohms it makes a received power N=U^2/R=(30E-6)^2/50=1.8E-11 Watts.

According the Friis formula in far field Ptrans/Prec=DtransDrec(lambda/4/pi()/d)^2 thus the distance provided by 25W giving those 12 dB of S/N ratio will be 560 000 meters ie 560 km. Probably I misapplied the D where I used 3 dBi for dipole so I needed to apply the multiplier for not dB form. If so, then 3 dB=1,4x so the distance will be 260 km. Anyway absurd because:

The same time radiohorizon d=3.57sqrt(h) between 10.5 m high mast and Earth is 3.57sqrt(10.5)=11.57 km. Thus between the two identical antennas the distance will be 2*11.57=23.14 km Thats all.

What a benefit gives to alter first figure limitation (with altering the D) if second limitation stays unchanged.

#### Tesla23

Joined May 10, 2009
488
Made a bit calculus to prove the said that antenna gain is not playing ANY role in UHF.
So, My MRF75D model have 0,30 microvolts sensitivity at SN ratio 12 dB. At 50 Ohms it makes a received power N=U^2/R=(30E-6)^2/50=1.8E-11 Watts.

According the Friis formula in far field Ptrans/Prec=DtransDrec(lambda/4/pi()/d)^2 thus the distance provided by 25W giving those 12 dB of S/N ratio will be 560 000 meters ie 560 km. Probably I misapplied the D where I used 3 dBi for dipole so I needed to apply the multiplier for not dB form. If so, then 3 dB=1,4x so the distance will be 260 km. Anyway absurd because:

The same time radiohorizon d=3.57sqrt(h) between 10.5 m high mast and Earth is 3.57sqrt(10.5)=11.57 km. Thus between the two identical antennas the distance will be 2*11.57=23.14 km Thats all.

What a benefit gives to alter first figure limitation (with altering the D) if second limitation stays unchanged.
The Friis formula is not appropriate here - there is all the water to consider!

You want the two-ray path loss model:

https://en.wikipedia.org/wiki/Two-ray_ground-reflection_model

in short, the path loss in dB between two isotropic antenna at heights h1 and h2 above separated by a distance D above a flat ground is

$Path Loss = 20 log_{10}(\frac{h_1 h_2}{D^2})$

#### SamR

Joined Mar 19, 2019
3,609
True, it does have a coax connector at the base of the balun. A good painting of several coats of white Sea Brite Liquid Electrical Tape will take care of that. Any dB claims are not for distance but rather for clarity of the voice signal. A couple of points about the homebrew. PVC is not UV resistant and will deteriorate. Painting may prevent that. Also the mounting issue that PVC is not very structurally sound and will be stressed at the clamping points. On my powerboats I use the 8' fiberglass antennas which do not have a bottom coax connector but the connected coax is only 20'. I have had them fail at the juncture of the stainless steel mounting ferrule and fiberglass tubing for the antenna casing. I'm sure they receive less of a whipping around than if they were mounted to the top of a sail mast. Which is why every sailboat I ever saw used the 3' stainless steel whip with the base balun instead at the masthead.

#### Janis59

Joined Aug 21, 2017
1,356
RE: Tesla23
Formula seems painfully known. Very probably if one calculate according it he would get the same result. Just I KNOW by own experience that UHF even the bad radio get the same distance what most ever seen brilliant radio and this distance is dependent only of both antenna H and nothing more
RE: SamR
When we used to install the LORA(TM) data links from tower to tower here, where I exposed me to be an antenna expert, we always use a special thermal rubber band. It is damn expensive, probably American, but You just glue-on like a chewing gum, bit warm it up, and it shrinks and simultaneously cures. So the plug is absolute water-tight and at least a first 5 years of system under Sun shows no a least signs of UV-degradation.

By the way, biggest confuse was when at one beer plant (actually best plant in my State, probably), so the dataloggers was into the metallic container and owner categorically declined we drill any hole in the walls. So I said, its a Faraday Cage, we MUST pull the antenna through the wall somehow. My mate said, yes, but at least we should TRY to have a data packets from inside with closed doors. We tried... and it worked. Not very well, the losses was some 60 dB but enough to have a fair datastream.

So later I thought and thought what was the reason before I realized - the Faraday Cage principle works unidirectionally, the main principle is - every charge what one put inside the Faraday Cup always is traveling to ouside a Cup and still inside voltage is zero forever. So, all the RF travels outside, but never is coming from outside to inside. Huh! Now I know.

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