Anatomy of an Antenna Feedline
Now that I have approval to install an antenna on the roof of our condo building, careful construction of the feedline(s) and control lines from roof to apartment should enable me to do this just once as access to the (~flat) roof is quite restricted – for good reasons.
By the way, since I’m expecting some non-hams to peruse this site, I’ve taken the trouble to explain some terms that many of you know all too well. Thanks for your patience!
Since we live at almost ground zero of the lightening capital of the world, I figure at least some reasonable measures of protection are warranted, without breaking the bank. First, as you all know, there is no complete protection against a direct strike. The objective is to minimize the likelihood by either disconnecting the path to the shack or providing a FAR easier low impedance path to ground than through your equipment. Since I have a second story shack, my strategy will be the former – disconnect – both at the antenna on the roof as well as in the shack.
On the roof, I’m installing “The Sheriff”, a gas tube lightening arrestor.
Should a direct or near hit spike current on the feedline, a small gas tube (the cylindrical device in these pics) blows like a fuse…
The gas tube completes the circuit against a gold connector in the body of the arrestor. This disadvantage is that a trip to the roof will be necessary if this blows. The advantage is that it should keep the strike well away from shack and equipment. During times of high electrical activity in the atmosphere, I’ll also disconnect the antenna feedlines where they enter the shack. For now, that will entail unscrewing the antenna feedlines from the back of the transceivers. Later, perhaps a switch directly to ground during storms.
Next order of business – the prevention of RFI (radio frequency interference).
There are two primary types of RFI suppression that I’m designing this shack for. The first is called differential mode RFI. I’ve installed a “low pass filter”, which should only allow desired frequencies to pass, and not “harmonics” or undesirable “echoes” of different (multiples of) frequencies I WANT to pass through.
The second primary type of RFI suppression in this design is called common mode RFI. At the radio frequencies I’ll be transmitting (HF, or high frequency, i.e., 14 to 28 Mhz; and VHF, i.e., 144 to 444 Mhz), I want to prevent that RF energy from traveling down the OUTSIDE surface of my coax cable, where it can potentially radiate to nearby appliances, for example, and contain it to the INSIDE conductor of the (shielded) coaxial cable. That is achieved by employing “common mode chokes” or inhibitors of this undesirable energy radiation within the frequency ranges I’ll be operating.
RFI suppression is achieved by passing the conductors of this energy through a series of ferromagnetic cores called “toroids”, and there is a different “mix” (composition of materials with varying properties) depending on various operating frequencies. See the following resources for more info if desired:
- A great tutorial on common mode chokes at http://freepdfhosting.com/1e5c787453.pdf
- A good description of RFI suppression with respect to how it affects audio g(and video) gear: http://www.audiosystemsgroup.com/RFI-Ham.pdf
- High level description of “beads” for power lines, speaker cables, etc, see http://www.eham.net/reviews/detail/5334
In anticipation of AVOIDING common mode RFI problems, I’ve been accumulating a collection of toroids, beads and split beads of various sizes and mixes. Here’s my “kit”:
There is a confusing array of fairly standardized part numbers signifying various sizes (for different size cables) and mixes (for various frequencies). I was confused, so I created this “cheat sheet” on the inside lid of my RFI kit;
For my HF antenna feedline, I’ll be using RG-213 50Ω coax, which has an O.D. (outside diameter) of 0.405″. That means I’ll need beads that are of the FB-102 size designation (0.5″ hole). And for HF, I’ll use mix 77. That means if I were to order these babies (I use Palomar Engineering or http://hexkit.com/rfchoke.html), I would get the recommended minimum six FB102-77’s which look like this:
Slide ’em onto the feedline cable and use shrink tube to hold ’em in place
Unfortunately, my largest shrink tube on hand wasn’t quite large enough to go over the FB102’s, so, never-say-die, I used on smaller bead on each end of this string (F82-77’s) which just fit into my largest shrink tubing, using these as retainers (you can always use more beads for more protection – OR to hold the others in place!)
Before sliding the ’82 onto the coax and shrinking, a nice tight fit (although this tubing is supposed to shrink to half its unshrunk size, which is quite a bit:
After shrinking with a heat gun – hair dryer would work too (nice, huh?):
The complete assembly. This will be immediately below the antenna up on the roof. If I experience any problems, I’ll first add another set where the feedline enters the shack. If any issues remain, I’ll add additional sets along the feedline at strategic locations (every 1/3 wavelength of my highest operating frequency):
Now the same routine for the VHF feedline, except for the “mix” of the toroids will be mix 43 for VHF frequencies instead of 77 for the HF frequencies. I just collected a bunch of 43’s from my kit with holes large enough to slide over the coax (another run of RG-213):
Assembled tools and materials. Note my heat gun is not in the pic – he’s shy because he’s so ugly from being so old and beaten up from hard shipboard use!
A clean coax cutter (blue handles) is necessary for good operation of the DX Engineering cable trimmer (in red protective cover below):
This cool little cable trimmer tool really eases the job of attaching connectors to coax ends in a quality fashion. Just insert the end of the cable and turn in a clockwise direction. Worth the forty bucks, in my humble opinion. Cuts in two operations. First cut in one end, second cut in the other… this one is for larger diameter RG-213 or RG-8 coax cables:
The first cut trims away outer insulation, copper braid shield and inner insulation, just to the right point needed for the standard (PL-259) connector that I’ll be using:
Here, the trimmer is trimming the second cut to bare the right amount of copper shield without cutting through it by trimming only the outer (black) insulation:
This is how the bared braid looks (nice cut that only requires a bit of further trimming, but for some reason, my first cut didn’t get all of the inner (white) insulation. i think I pushed the cable too aggressively into the end of the trimmer as it left the insulation around the inner conductor, and it should not have done so. The next one, with a little less pressure on the cable into the end of the device came out perfectly:
Then slide the barrel of the connector onto the cable, and then turn the other half of the PL-259 onto the cable end. When it got tight (a good thing), I needed to employ a pair of plyers and a golf club to get the right grip for completing the turning on process:
Before soldering (four holes in sides and pin at the end):
Below, after soldering (use thin resin-core solder, not acid-core).
The trick is to get the connector hot enough to encourage the solder to flow into the holes without melting the white insulation underneath the copper braid shield, which would have the undesirable effect of melting the outer shield to the inner conductor, creating a short!
Testing for a short – nope – guess I didn’t screw THIS one up, too badly, anyway!
The finished product (at least this end of the feedline). The other end of the feedline will receive no connector or beads until after run through the conduit:
Other control lines (this is for the antenna rotator) will get wrapped around the appropriate beads or toroids (more wraps = better RFI suppression) at both ends of their length as well:
And here’s the collection of wires and cables that will connect roof to shack:
- HF feedline (one line for four bands – 20, 17, 15 & 10 meter)
- VHF feedline (one line for 2M and 70cm)
- Rotator control line
- Antenna coupler control line (and spare feedline)
- Strong parachute cord for pulling additional wires back and forth through the conduit “just in case” (for adding more RFI suppression if needed, for example).
Spools ready to lay out for installation, hopefully within the next couple of days:
The two inch conduit through which all this stuff must pass (including sufficient room for potentially adding more RFI suppression beads as required). Using elbows with opening access panels to ease cable runs. note all are painted to color-match the exterior paint on our building. Should be pretty “invisible”.
Well, that’s my prep for feedline installation. In future posts, I’ll add pics and descriptions for rooftop antenna installation and how all this stuff will then enter the shack.
73 (best regards)
de (this is) K0GKJ (yours truly)
dit dit (traditional sign-off between CW (Morse Code) operators)