Are you in a situation where your pirate radio station ham radio shack has insufficient land for an HF dipole, but you're well up for pinching the neighbours' roof?
Perhaps you've already deployed an end-fed wire over their tree. But you're having trouble getting a good earth? Or there's too much QRM from the earthing arrangement?
Or the neighbours' fillings are arcing when you go above 16kW peak envelope power, but you can't muster up the extra transmit power to put an end to the screaming forever?
You need the end-fed dipole!
First of all, it's not my own design. At least as far as I know. I simply took the well-known Flower Pot Antenna, removed the solid components and tipped it on its side.
Someone must have done it before. Probably called something else.
Deployed in the field:
Splice to fishing line
Coax shield | ___
removed from here | / \
| | / \
| | \ / <---- Throw-weight stuck in tree
RF choke | V _/\ /
_ | | ___/ | |
/ \ | V ____/ | |
/___\ V _______/ | |
| _ |___/ | |
| | | | | / \
+-+-+-+______|_______________/_____\_______
^ ^ ^
| | |
Love Shack | Neighbour's
| tree
Fence,
property line
What the neighbour doesn't know won't hurt them. At least below 20 kW.
Schematic view:
RF choke
| 1/4 wavelength
50 ohm coax feeder V +-- coax length --+ Fishing line
| __ | | +-----+
V | |V V | | __
+--------------------| |------------------- V V| |
| +------------------| |--------------------------------------##-------| |
| | +----------------| |------------------- ^ ^ |__|
| | | |__| ^ | | ^
ATU | +---- 1/4 wave ----+ |
| | | Shield removed centre conductor |
| | | <- 50 ohm coax from here only |
| | | Throw
Radio weight
The RF choke should be a few turns around a type 43 toroid, but other arrangements will work too. Keep the turns spaced out to reduce capacitance.
You may be able to use clip-on ferrites designed for power cables. You could also just make several turns with an air core, like the original Flower Pot design. You want the impedance at your operating frequency to be as high as possible. A few kOhms if you can do it. If you can't, your ATU will have to work harder (make sure you use a fairly low-loss core material in this case).
From the midpoint of the wire, the shield needs to be completely removed. You have three options:
The very tail end of the wire is tied to a piece of fishing line and a throw weight. This lets you get it up on a roof or in a tree. The fishing line keeps the end of the conductor away from any leaves or structures. You want some separation particularly at the end, because the impedance at this point is high (and the voltage high also).
First, take an ordinary centre-fed dipole, with a choke used as a balun:
First half of dipole, Second half of dipole,
connected to coax connected to coax
shield core
| |
V V
-----------------+ +-----------------
| |
_ | | |
| | | | |
----------| |----+ | |
----------| |------+ |
----------| |--------+
^ |_|
| ^
| |
Coax feeder |
|
RF choke
We want to end-feed it, so replace the first half of the dipole with thick copper tubing. Then run the feeder up inside the tube and connect at the centre-point as normal:
RF choke Coax inside tube
| |
V |
_ -------------|--------+
| | V |
----------| |------------------------+
----------| |-----------------------------------------------
----------| |------------------------+ ^
^ |_| | |
| ----------------------+ |
| ^ |
Coax feeder | |
Large diameter Centre conductor,
copper tubing, second half of
first half of dipole
dipole
Obviously this will still work, you've just hidden the coax feeder inside the dipole itself. But we can do better still.
To understand how we eliminate the copper tube, you must recognise that a coax actually has three conductors.
What?!
Yes, because of the skin effect and the inductive coupling between the centre conductor and shield, separate currents can flow on the inside and outside of the coax shield!
Of course, only at radio frequencies.
It looks a bit like this:
Cross-section of coax, from the perspective of a radio-frequency signal:
---------------- <--------- Conductor 1: Outside of shield
/ \
/ ---------- \
/ / \ <----------- Conductor 2: Inside of shield
/ / \ \
| | _ | |
| | | | <--------------- Conductor 3: Core
| | |_| | |
| | | |
\ \ / /
\ \ / /
\ --------- /
\ /
----------------
Imagine we connect the radio to a dummy load using a piece of coax. Current leaves the radio through the centre conductor and travels to the dummy load. The current returns using the inside of the coax shield.
Because these two currents are perfectly balanced, no current will flow on the outside of the coax shield! It may seem surprising but it's true. Everything that happens inside the coax in terms of send and return currents is entirely private.
So if we take our copper tube and remove it, what will happen?
Simply put, the outside of the coax becomes the copper tube. The inside functions exactly as before!
The exposed centre conductor making the second half of the dipole will "work against" the outside of the coax shield. The outside of the shield becomes the other half of the dipole.
Currents on the outside of the coax shield cannot travel back up the feed line and into the radio because of the RF choke. So instead they must flow towards the centre of the dipole. From there they flow in opposition with the currents on the centre conductor, on the inside of the coax shield.
Electrically, it's almost identical to running the coax inside a copper tube and connecting it all at the mid-point. Strange but true!
You must place the RF choke at the proper location. Exactly 1/4 wavelength from the point where the shield is stripped back. If you place it at the radio, you've made a very lop-sided dipole with the feed line acting as a very long half! It will work, but your radiation pattern will be all messed up because of the improper length.
Of course, RF chokes aren't perfect. The feed-line will always contribute something to the antenna system with a setup like this. You may benefit from an extra RF choke before the feed-line enters the shack. It won't improve the antenna system much but it might keep RF out the shack.
At high powers, the voltage across the RF choke will be pretty high. Several clip-on mains-cable-style chokes are a better bet if you're running proper power. Or a physically long air-cored coil. Watch out for ferrite cores getting hot.
The situation is worse than a centre-fed dipole. The RF choke is at the end of the dipole where impedance is high and voltage is high also. This means your RF choke has to be a fair bit better than you'd use for a centre-fed dipole to eliminate RF in the shack.
It sounds more difficult than it is. For modest power it "just works".
We all love a good Chinese spy balloon, so why not get in on the fun?
Maybe your licence prevents you operating from a balloon? No worries, you're operating from the ground, you bell-end!
The ease of setting up a long vertical using a drone* or balloon is very appealing. But traditionally the problem is the grounding. It takes 100 times longer to put in a half usable ground system than it does to fly a wire up!
The Flower Pot antenna design lends itself very well to hanging from a balloon or drone* (remember: not operating from. You're not allowed to do that)
Obviously weight is limited, so use the very tiniest coax you can for the first half of the dipole. The stuff used for WiFi antennas inside laptops is a good bet. The losses will be minimal at HF frequencies.
* When you hang an antenna wire from a drone, it isn't a drone. It's a fan-stiffened antenna. Most vertical antennas aren't stiff enough to stand alone, so they need help. Some people use guy wires. Or you can use a fan to keep it stiff. No it isn't a drone! NOT A DRONE!! NOT AN AIRCRAFT!! NO!!!
No.
Also yes.
It's isolated by an RF choke, so it doesn't have the RF-in-shack and QRM problems of a counterpoise wire connected to the ground screw of your radio. It also has a more predictable radiation pattern than a random bit of ground wire because the length of both elements are known and properly positioned.
You can call that a counterpoise if you want. Means anything really.