HAM RADIO: Antenna Autopsy 2 A look at a 2M /70CM X300 Diamond/Watson Antenna 

Love it! Time to strip down "You Know What" next! See you at the factory!! LOL

Before I was old enough to attend university for my engineering degree I learnt so much by dissecting bad electronics and figuring out why and how it worked. Now, 40 years later and after a career as an RF engineer those memories and experience continue to shape my professional and personal life. I remember scraping the coating off of the different styles of resistors and finding some that were just pure carbon, other that were glass rods that had a circular track grooved in to them with conductive powder in the grooves. Taking apart capacitors; Even breaking open tubes to look at the plate, anode, grid and filament structures. Invaluable to be able to do that and make sense out of how things really work.

I've heard capacitors get old and can become resistors. I really don't understand anything about that yet, but I'm interested in figuring it out. I like this type of video because it's very informative and that is how things are figured out by troubleshooting and experimenting and experience is everything. Great video. Thank you

The centre pin of the base can also go open circuit from the wind flexing it so good to hear FM repeaters in the background a lot have now gone

You really are a good teacher, er “ELMER”! Thank you!!

I blew a similar antenna up a few years ago running 200W on 6m into it. I still have it so should really try and repair it and replace the ceramic caps with coax caps - probably be good for a LOT more power than the rated 100W,

You repair stuff the same way I do! I'm so glad I'm not the only one!! 😂

Great video, very instructional, i didn't know about the coax capacitor. 73, M7AXN

Oh! Watching...!!

I believe that their is a "cap" at each of the funny antenna bend sections, 3/4 wave points. Thanks again for sharing, always learn a bunch here.

That was excellent trouble shooting. Great hack making your own 6pF capacitor. You Brits are so inventive and resourceful. Thanks for the magnetron by the way.

Hello Mike, interesting to learn that someone else has this problem. Sorted mine with silver mica capacitors in place of the ceramic type. Initially thought it was going to be a bin job until I saw the Allen screw holding the base in place. Retired from the radio Engineering Industry now, so plenty of time to play with the ham radio. Single feed to two radiators, current at different frequencies will flow out of the junction where it has the least impedance same as the nest of dipoles, ie if it sees 50 ohms it will flow into that radiator, that is one of the reasons we never used multi-band aerials in my early PMR days, too much hassle if a fault developed. vy 73 de John G4YDM

👍Well done Mike. I cannot say I am very surprised the little disc ceramic failed. Hopefully, the co-ax capacitor will do a better job. I am guessing that the tapped coil at the base is an autotransformer used to drop the impedance resistive element down to 50 Ohms and the shunt/series capcacitors will be cancelling out the reactance to around 0 Ohms. This means that you are left with approx. 50 Ohms restistive presented to the transciever. The "bendy bits" along the copper vertical section will be to adjust the phasing from each lower 5/8 element to the one above. They are bent back on themselves to reduce radiation. The idea is is the phasing sections don't radiate but just add extra length to make sure the 5/8 sections all radiate in phase. This is what gives the colinear it's gain.

great mike. found the problem, a bad 6PF cap. what happened that cap got so hot it un soldered it self. that a colinear antenna. the coils up the antenna push the lobes out on the pattern flatting the donut pattern. nice cap. if you get full data sheet on coax will have cap value per meter listed. gives you a cut chart. the 5/8 wave gain master for 11 meters uses a coax cap inside as well. 73's

Hi Mike, thanks for another fantastic video!! I always find your videos extremely informative, interesting and helpful! I often use your video on calculating Q, reactance and inductance as a teaching aid to help explain impedance and resonance. Keep’em coming! This is true ham radio!

Very interesting to see the antenna internally thank you for sharing.

At 1st glance, I would say it’s stacked co-linear arrays. Something to read up on! 😊 de WR9R

That first close up of the cap was a clue. I noticed that the exterior cover was broken where the leads enter the head. That made me think you might have a bad cap. Given the apparent age of the antenna, I think replacing them all would be appropriate. Nice bit of diagnostic work.

Thank you sir for sharing this. By the way, clever idea on clay. All the best.

Nice job Mike 🙂👍

I had also same issue with a 3 band antenna. It save 300 watt max power I use was 150 watt on 2 meters. But most of the time it only was 25 watt ARPS. I fixed it 2 times. After I only use max 100 watt I had no problem more.

You have no idea the trouble I would be in using the kitchen worktop as a work bench ! 73 M7BLC

Hi Mike, G6BPC in stroud here. I did exactly the same mod to my X510. Not because it was faulty, but there is evidence that the capacitors are prone to melting at 100 Watts or more. I think it's my most popular video! Mine is an original Diamond and after many years the three seals on the sections have not let in any water. Thanks, Nick

Very interesting to see the internal construction of the aerial, when you tested the small capacitor it seemed OK, perhaps it was breaking down when it got a higher RF voltage across it. Good tip using the coax

Great video, well done.

It is a common problem these capacitors often fail. Either due to static load or excessive RF. I am curious how you routed the coax capacitor inside the antenna housing. Best 73, Stan (DK2CA)

Glad you’re sorted Mike 👍🏻 Not sure how old it is, but my two Diamond X-300s, 5 years and younger, don’t match yours, so possibly something else, unless it’s some sort of older design 🤔

So the conclusion is the capacitor was broken even though the LCR meter initially sowed 6 pF? Or am I misinterpreting?

Thanks, i have the same antenna.

Great fix the antenna will handle more power than original.

Wouldn't transmitting FM on 144.000 be transmitting out of band..? I could say the same for 146 but you may have a NOV..

Wa6ols Wes. It's a colonial vhf/uhf with a matching circuit.

It’s a collinear antenna. Those folded sections are just quarter phasing stubs. They can be folded or arranged in various ways. Some are just a U shape horizontal or wrapped around in a circle. Look at a super-j or marine Shakespeare antenna.

Awesome job as always but i would have replaced all the caps with coax while it was apart just for piece of mind and for it to handle higher power

What's up with the S-7 to S-9 noise floor on 2M? Hope it's not like that once you get it back up in the air.

It's because your kettle was the wrong inductance, it was out by about 500 Pico Kettles 🤣

I have the same antenna with the same problem, once i get it down, this video will be used for relevance, Just one question Mike, if the original capacitor was OK and just lifted off the one leg, why did the SWR not come back down? mine is high on 2 and 70, brand new feeder and plugs fitted. Found a short length of feeder the same as yours, no idea what i did but it measured as a resistor not a capacitor 0.04A. shoot me now Mike, it could only happen to me. Just for clarity, the feeder you used as the capacitor, is it a specific length overall? the one end looks like you bared the braid and pulled it back and then just bared the centre conductor yes? i did this and couldn't get a measurement. i take it the other end is just cut and left, not joined?

so the original cap was broken even you tested/measured it? or what was the reason it finally worked? sorry, english is not my native language

Quite disappointing to see such an ordinary looking capacitor used in a power RF situation.

Sir, how did you see that the coil was overdone by one turn? I”m sort of a nube to this…

I wish to know How to fix my antenna, no body in my country know how to fix 😔

in the kitchen u got the radiation element directly connected to the radials /ground . its litterly leaning against the part at 11:33 how just how

Running a kilowatt will smoke those ceramic caps. I promise.

I think the caps are too small.

make a j-pole

My wife would kill me for working on a real wood kitchen worktop!!

You need a workshop.

Certain sure that's not a Diamond got 3 of them here. Diamond have the larger radials. Had this happen to me one before using a Sharmans copy X300 that was used on a remote site with high wind loads. It was only used as an RX antenna so not sure why the capacitor went in it. Whilst I also had it apart I added plenty more foam inside of the GRP shroud to stop it rattling, its lasted 6 years since.