You are right, but for the purpose of making a CoCo, such finer details are unnecessary.
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Andrew Heliax LDF is very costly and not sold by common stores. It is rarely used for making a DIY Coco. A hobbyist who lives in my area did make a Coco from LDF1-50, but only after he got a length of it free of cost from a friend. Here are two photos of his 14 element Coco made of LDF 1-50 Andrew Heliax cable (the one with black outer jacket). The other made of brass tubes is a commercial antena for Wifi
The Andrew Heliax has a corrugated shield, which effectively increases shield’s length as compared to a plain shield. This makes Adrew-Heliax’s radiator closer to half wave than the radiator of a standard plain shielded coax.
Local policeman: “What are you doing here rummaging around the communications tower ?”
Me: “Collecting wire”
Local policeman:" Did you cut that stuff off the tower intending to steal it ?"
Me: " No, they left scraps around"
Local policeman: “What do you think your doing with those ?”
Me: “Tracking airliners”
Local policeman, becoming alarmed : “Excuse me, did you say tracking commercial airliners ?”
Me: "Yes, I can see exactly where they are and where they are going and… "
Handcuffs. Dragged off never to be seen again.
Joking of course, but I think unless someone cleared me picking them up not something Id personally go hunting for…
The same applies to other innocent hobbies, like geocaching. Not a problem in Canada, the US, or Western Europe. But in some other countries, the last thing you ‘need’ is to be caught with a GPS receiver, checking behind bushes, inside holes or digging. Ever heard of ‘dead drops’?
Then there is/was Pokemon Go. The places people would go, oblivious to anything else. There is an ‘iconic’ picture of a guy in an area where the big sign clearly said: Danger. Stop. Do not enter. Mined field.
Sorry for the off-topic. Back to our regularly scheduled programming.
A length of coaxial cable radiates negligible amount of RF.
Same length, if cut into pieces of length (λ/2 * VF), and these pieces joined again cross-connected (i.e. core of a piece to shield of adjacent piece & vice versa), then this length of coax becomes a very good radiator of RF. Why & how?
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The shield is the radiator. It transmits the RF power fed to Coco, or recieves arriving RF signal intercepted by it.
The core wire being shielded, does NOT radiate any RF power, nor it receives arriving RF signal. Its primary funtion is Phase shifting the signal by 180 degrees. That is why its ELECTRICAL length is 1/2 wavelength. This phase shift is essential so that currents in shield of each piece becomes in phase with the currents in shields of adjusent sections.
A few questions that display my depth of ignorance or lack of knowledge- I’m trying to take it all in and frankly my learning is far from where it should be.
A ) Reading about the Skin Effect had me thinking about the physical construction of the coax collinear antenna. Should the elements be joined in such a way that the center conductor is firmly in contact with the inner surface of the braided shield between the dielectric and braid instead of between the outer edge of the braid and outer insulation ?
B) Reading about surface roughness and effect on Skin Effect, I wonder how much each nick and cut made to the center conductor to strip the dielectric and braid affects efficiency of the antenna ? The copper clad conductor coating is very very thin. If I’m heavy handed and circumcise the wire with a knife, I have a thin copper skin, a small steel canyon, and thin copper skin at the end of every element…
Could it be that some small construction details are what defeats success with these antennas - as much as we focus on trimming to length and velocities etc. I confess to speeding through other aspects that might be equally important.
I dont think skin effect has any relevance to Coco’s construction or performance.
The core wire+inner surface of shield forms a transmission line, and both the core & the shield’s inner surface have equal and oposite currents flowing through them.
The outer surface of the shield intercepts the arriving RF signal, and the current generated in it by arriving RF depends on the RF signal strength.
If the coax (out of which coco is made) is good for rf signal flow through it, the coco should also be equally good for signal flow.
It may not have any relevance at all at 1090 MHz, regardless of antenna type. More surface area, increases the bandwidth. An ADS-B signal occupies only 50 kHz. If the ADS-B ‘band guard’ is 10 MHz, for example, it still is less than 1% of the carrier frequency.
Exactly this.
Which gets back to the desirability of the Vf being high so the physical length is closer to a free space half-wave resulting in an antenna (section) closer to resonance.
After applying Velocity Factor, the Physical Lengths of core and shield are both less than half wavelength by a factor of Vf.
However the core wire being surrounded by insulation and shield, it’s Electrical length is exactly equal to 1/2 wavelength.
The shield is in open air, and therefore its Electrical length = Physical length which is less than 1/2 wavelength by a factor of Vf.
When Vf=1, the electrical length of shield = physical length of shield = 1/2 wavelength. Now both the core and shield have electrial length =physical length = 1/2 wavelength, an ideal situation.
Can you guys help me understand why a damaged ( nicked) center conductor would have no bearing on performance ? It cant be a good thing…I think I understand the convo about Vf and ideal resonance ( which I will never achieve LOL)
If you use push-pin solderless joining of pieces, the nicked center conductor will badly damage the braid & foil of the piece in which it is inserted. This will ultimately lead to a broken connection.
If center conductors of pieces are soldered to shield/braid of adjuscent pieces, the nicked conductor will not do any harm.
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Forget about all these details. The Coaxial Cable Manufacturer has already taken care of these, and user dont have to worry about these design and manufacturing issues.
Oh, yes. I did not think of it this time, but I mentioned previously about poor connection as one of the reasons for the possible ‘hit and miss’ with Co-Cos using push-pin solderless joints.
I wonder though, if the conductor is of the copper clad type. A nick in the copper cladding could/would affect performance?
My stock whip, which came with DVB-T Dongle, is made of steel wire, and no copper, brass, nickle, silver, or gold plating. Still it works wonderfully well.
