Can My Feedline Act As a Radial (
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- Опубликовано: 12 мар 2024
- Lou, N9LTW, was wondering how far his ground radials should be from his coax. He is concerned with getting RF in the feed line. Watch to learn more about his interesting idea and how It could turn out.
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No No No... bury your radials if you have anyone using the back yard for other activities. This includes your family dog that likes to roam the yard. It's more of a "practical thing" than a performance thing!
I'm not sure what frequency this is for... but I run several Broadcast Services on MF & HF using 1611 khz and 4970 khz.... Both are ground mounted Inverted L's. I've had all sorts of drama's with "Common Mode Currents. It took me a few "heated discussions" with HAMS that can''t grasp the different needs of a Broadcaster. These are low power and are in close proximity to each other (they share the same earth mat). I need the tuned dip to be as wide as it can be, so I get the best audio bandwidth that the aerial Q will allow. I also want some sort of "rejection" offered to the opposite service.
I buried my radials because we have a dog, only about 4-6 inchs down. We are lucky to have VERY good dirt under us. You can disconnect the radials and see a drop of about 10% in coverage on 1611 khz. I use to run the coax on the ground and I did let the grass grow over it (as Dave mentions). Thankfully it was only a short run. But boy did I have tuning problems!
It took me some "very heated discussions" as mentioned to discover what was going wrong. I'd get the analyser readings R & J etc and punch them in to different modelling software. MMANA was the main one used (a couple of others point out things MMANA doesn't). I knew about High Pass and Low Pass as I wanted rejection of the oppsite services (as mentioned). MMANA has a LC matching tool, use it with the readings you get not the ones "it thinks" the aerial is when you put in all your dimensions. When you are told the values of L and C needed to do a simple L network you then need a Series Complex Load that represents the R & J of your aerial. If you do it this way you can get everything right on the bench against your mock up aerial load. Winning right? Nope!
I then take it out and put in the ATU box and the tune is WAY OFF! What gives? More heated discussions, Peter why are you using a L network? You said you wanted wide as possible. Which I did! But I also said I want rejection of the opposite service too (insert pic of ham with puzzled look). He was telling me to use a balun come matching transformer (that's another topic). But just not done in broadcasting circles. What does come out of this conversation is I get a much better understanding of Common Mode Currents.
My original fix to everything being different when the network was dropped in the box was to add a coax extension so I could have my Rig Expert (analyser) physically connected to the coax going to the TXer (all in situe in the ground with other coaxes and radials). This worked.... initially I thought I was on a tuned length of coax and it was effecting the matching. But silly me didn't wake up to the fact that a couple of times my "extension cable" use was different (I have a few spares). You'd think this would make the penny drop. Different lengths "should" move the resonance of what was a stub (that's another story too). But no... same problem. In fact it would move the tune by about 80khz no matter wich cable was used to extend it.
The penny finally drops.... I'm watching a video (not dave's) on Common Mode Currents. Oh! I wonder if that's what is happening here? Then ole mate says... one of the things CMC does is make your aerial look wider in tune than it really is.... OH! Penny Drops!
In broadcasting the coax is buried down at 600mm the same depth as electrical cables (by law here in Australia). This offers "isolation" from the earth mat...where your radials are only a few inches down. I'm starting to form a thought in my head... You know what... how does the RF know that a run of coax buried at the level as the radial - isn't a radial? Answer... it doesn't! So we need to help it!! Enter something like an Ugly Balun or we now step into the whole world of Toroids and different Ferrite mixes (another world of heat discussion with HAMS). Keep in mind I'm always on, this is not 30% duty cycle. It's not big power either, so some ham tricks may work.
I'm on 1611 khz and 4970 khz with 100W always on AM power. Some of the Toroidal designs will work well on 5mhz but totally unload due to the core mix on 1611 khz. I learnt this lesson the hard way. The super short version here is crack out an Ugly Balun.... 16 turns on 100mm OD pvc pipe made with RG213 that will cop decent power. The rule here is you want about 4 times the Reactance Ohms on the outside shield to the 50 ohms (seen on the inside of the coax). So the currents on the outside of the coax get blocked.... In my case 4 times is at least 200 (I get 380) on 1611 khz with something near 1800 ohms on 4970 khz... in short it works VERY WELL!
A test I do now is with any ATU installations made with ground mounted earths is as follows.... Hook up the analyser to the 50 ohm point without any part of the coax connected. Just the aerial... and read the SWR. Then grab the shield of the coax and tap it to the earth bar where you earth everything. If you see this changes the SWR, it's clear your coax is being seen as part of the aerial. You definitely don't want this.
I could have possibly made this comment a lot shorter and said - bury your radials and install a Common Mode Choke at the feed point. But I figured some context was needed.
73's all
Thank you, Dave.
Still have lawn staples left. Got a deal on them in the fall.
Have a great day. N0QFT
If you have a lawn edger you can easily make the grooves you need if you want to get them protected immediately. At a minimum using a weed whacker to clear a groove to get down to the ground will typically do. The only thing is, I would bury coax since it's susceptible to damage. I like to bury pvc pipe. That way if I want to mess with things like different coax etc I just pull out the old and pull in the new all at the same time. Cheers.
Thanks Dave!
That's how "no radial" antennas work.
How come when your antenna needs radials and it is on the ground you need 30 and when the vertical is on the roof you only need two?
ASK DAVE, HOWS LIFE TREATING GUY 👋
So grab a pencil and run to your bathroom. Hold your pencil ninety degrees onto your bathroom mirror. Now look, you see not one but two pencils one in your hand the second is the image in the bright shiny mirror. This is exactly what your radials do for your vertical, it provides the missing half. Now the more radials you lay down the brighter and symmetrical your image will be. So do you want a dirty fogged up mirror or a bright shiny one.
Well aren't you special!