Back Feedline, Connectors, Coax and more...
Compiled by Mike Morris WA6ILQ

First of all, despite what you read elsewhere, "hardline" is not Heliax™ and Heliax™ is not "hardline".

True hardline (sometimes called rigid feedline or rigid line) is most commonly used in broadcast (AM, FM and TV). It does not flex - it's based on concentric pipe with an insulator between the inner and outer conductor. From the outside it looks like flanged pipe. Common sizes range from 3/4 inch to over 8 inches in diameter. It is much lower loss than coax or even Heliax.

Courtesy of David Tubergen, of T&W Electronics Inc. of Grand Rapids, Michigan we have three photos of a 24 inch (61 cm) length of six inch (15.25 cm) diameter hard line made by Dielectric Corporation.
Photo 1 is a view of the entire 2-foot line section. The black "base" in the photo is in reality a protective snap-on plastic cap that protects the flange surface during shipping.
Photo 2 is the male end that plugs into the next length. The black plastic ties are for shipping, they keep the white insulator in place. On the left side of this photo you can see the protective black cap on the flange on the end of a second length of line.
Photo 3 is the female end (the black plastic cap goes on this end).

As you can see, there is a difference between Heliax and hardline.

Kevin Tubbs KD2SL has additional photos of hardline feedline on his web page that describes how he recycled the retired WSTM-TV (channel 3, 60-66 MHz) analog broadcast antenna system in Syracuse NY into a 6 meter repeater antenna. This antenna weighs over 7,000 lbs (3175 kg) of 8 bay batwing at 850 ft. (260m) and about 1000 ft. (305 m) of 3-1/8 inch (8 cm) rigid copper feedline with about 0.7dB loss in the entire length!

Since hardline is so rare in the amateur radio and land mobile radio world, in this article we are going to focus on coax and corrugated copper feedline, commonly called Heliax™, which is a trademark of Andrew Corporation.   Just like all brands of facial tissue are commonly called Kleenex™ you will find folks calling any corrugated copper feedline Heliax™.

The feedline and the connectors (and how well you waterproof them) can make or break a system, and both of them are usually ignored forgotten until they cause problems. And problems with feedlines - coax or Heliax - can get real expensive real quick.

In duplex service you want to avoid any coax that has dissimilar metals rubbing against each other (such as Belden 9913) or any LMR-(any 3-digit or 4-digit number) cable since both use an aluminum foil shield rubbing against a copper braid (and they are not the only ones with that type of construction). In a coax cable any dissimilar metals in contact with each other are bad news. Aluminum oxide is formed when raw aluminum is in contact with oxygen, and the chemical reaction that converts the top few molecules of the exposed surface of aluminum into aluminum oxide is almost instantaneous. Aluminum oxide makes a dandy diode. All those millions of contact points between the copper braid and the aluminum oxide layer on the aluminum foil become millions of little tiny diodes. In the presence of high RF power levels all those little diodes cause RF noise. The amount of noise energy on any one frequency (such as on your repeater input frequency) is a low level, but when you have the noise source inside the same feedline that feeds a sensitive receiver it doesn't take much level to be audible.

I repeat - any cable that has dissimilar metals pressed together, even inside a jacket, will sooner or later create wideband noise (sometimes called duplex grunge) when hit with RF power. Even something as simple as a 1-foot-long jumper between the feedline and the antenna at the top of the tower can cause major desense problems (and for a long time the major antenna manufacturers were shipping 9913 jumpers with their antennas). 9913 is usable in an indoor simplex environment, but you will find that 9913 or any dissimilar metals cable, especially LMR-(any 3-digit or 4-digit number), is a disaster just waiting to happen on a duplex system.

Aluminum foil shielded cables in an outdoor enviroment are just plain bad news for another reason! One of the problems with aluminum foil jacketed cables is water. More than once a not-quite-perfect waterproofing job on a connector at the top of a tower has let rain, snow melt or ice melt water into the cable.
Example: At one repeater site someone used a length of 9913 to connect a VHF ground plane to a 2m remote base radio connected to a UHF repeater. Within a year the aluminum foil shield turned into white powder inside the outer jacket, and the copper braid was green and black. The reason? It was a simplex radio, someone didn't read the data sheets and didn't waterproof a connector. Belden 9913 does have some performance advantages over the 70+ year old design of RG-8, including a lower per-100-feet loss, but the other parameters suffer. Belden 9913 was originally never intended for outdoor use - it was developed as an indoor-only RG-8 sized low loss data cable (the old "thicknet" Ethernet cabling). The lower loss is achieved by using a low-density foam for its inside dielectric and a slightly larger center conductor. It uses aluminum foil under a tinned copper braid for shielding. Since then other manufacturers have duplicated the cable, some have changed to a tougher outer jacket, and their data sheets rate it as indoor / outdoor cable.
When selecting a cable many installers tend to look only at the loss figures while ignoring the other parameters - and the compromises that have been made to develop one cable over another. One of the compromises in the original Belden 9913 is the low density foam that allows water to migrate inside the cable down a tower run... 9913 is known as "water hose" in some circles for a good reason - and it's not because of the appearance.

In short, LMR-(any number) is usable in simplex operation on HF, VHF or UHF, but never for duplex operation. And personally, I consider 9913 (from any manufacturer) as an indoor-only simplex-only cable.

On the other hand, RG-214 has two silver-plated braids and a silver-plated inner conductor. RG-393 is similar but has a Teflon™ outer jacket. Both are the larger diameter (i.e. RG-8 / 213) size coax and RG-142 / RG-400 is the smaller diameter (RG-6 / RG-58 / RG-59) size. Please save yourself some grief and use real Mil-Spec RG-214 or RG-393 as the jumper between the feedline and the antenna. And please be a very annoying and nagging perfectionist about the installation and waterproofing of all feedline and jumper connectors outside the building.

In an email to repeater-builder, Eric Lemmon WB6FLY wrote:

Genuine, MIL-C-17 RG-214/U coaxial cable has double concentric silver-plated copper shields. Several companies manufacture an RG-214 "TYPE" (or "LIKE") cable that is very similar, but without the silver plating. As you would expect, it's a lot cheaper than the genuine RG-214/U stuff. Such "TYPE" (or "LIKE") cable may also have other differences, such as less braid coverage than the genuine cable.

Genuine RG-214/U has to meet the Mil Spec Number MIL-C-17. If the manufacturer is going to ignore one part of the specification (the silver plating) what is preventing him from ignoring another part (the outer jacket UV resistance, or the braid density, or the purity of the copper in the center conductor or braid)? If you don't see "RG-214/U" and "Mil Spec Number MIL-C-17" printed on the jacket, it's not real RG-214/U cable.

You will minimize your feedline problems if you stick to Superflex, RG-214, RG-393 or RG-400 on the radio side of the duplexer, and Superflex, Heliax and RG-214 or RG-393 on the antenna side of the duplexer. Use RG-214 or RG-393 as the jumper between the duplexer and the Heliax, and between the Heliax and the antenna. Note that RG-400 has more loss than the larger diameter cables - it's spec'd at 9.6 dB of loss per 100 feet at 400 MHz. It's short jumper material, not feedline material! More details here and on the RG-400 data sheet.

You will want to use all silver plated connectors and absolutely minimal adapters (and those need to be silver plated) in your connections from the transmitter or receiver to the duplexer, from the duplexer to the feedline and from the feedline to the antenna. NO nickel plated or chrome plated anything in the RF path anywhere! Nickel and chromium are ferrous metals and as such both are an intermod creator anywhere around RF. It's detectable at 10 meters and 6 meters and has proven to be a real problem at 2 meters, 220, 440, 900 and 1200 MHz. Mark Abrams WA6DPB said it well when he said "One nickel or chrome plated anything can really ruin your whole day". Another rule is nothing but Teflon™ insulated silver plated connectors and minimal adapters on anything above 30 MHz.

You will want to be a very annoying, nagging perfectionist about connector installation and waterproofing. Let's say that you have 100 watts of RF power coming out of your transmitter. A 1 dB loss in an improperly installed connector will cost you 20 watts. That's right - even 1 dB of total insertion loss is 1/5 of your power, 1.3 dB is 1/4 of it, and 1.75 dB is 1/3 of it! Just 3 dB is half of your power. The purpose of the repeater is to communicate, not be a space heater. With this kind of loss, the connector will heat up very fast and eventually fail, and the heat may damage the coax first.
And don't forget that losses are both directions. In a duplex environment with a single antenna having 3 dB of loss in the feedline and duplexer (i.e. everything between the antenna and the receiver) will cost you half of your receive signal. And if the problem is a bad jumper between the Heliax and the antenna (i.e. at the top of the tower) you can tweak the duplexer or tune the receiver all day and never fix it.
So to repeat myself, be a very annoying, nagging perfectionist about connector installation and waterproofing. And always favor the system receiver.

Use a right-angle connector rather than a normal connector and a right angle adapter. If you absolutely HAVE to use an adapter, always use a silver plated adapter (Kings and Amphenol are the preferred brands), never chrome or nickel. Both are ferrous metals that can cause intermod. And make sure you replace that adapted cable with the proper one on the next radio site trip.

In another email Eric Lemmon WB6FLY wrote:

I personally have settled exclusively on crimp-type connectors made by RF Industries, and I swear by them. Whether N or BNC, I use only the silver-plated connectors with gold-plated center pins and Teflon dielectric. It helps to have the proper tools to strip and crimp each connection, and I am very pleased with the quality.

RF Industries main web page is at http://www.rfindustries.com. Their connector page can be reached via the main page or at http://www.rfcoaxconnectors.com.

Don't go cheap on the Heliax feedline. Look at the dB-loss-per-hundred-feet numbers before you shell out your dollars. Personally, most of my runs are in the range of 90 to 175 feet. Long ago I decided that for UHF I was going to stick with a minimum of 7/8 inch and for 1200 MHz it was going to be at least inch-and-five-eights. You can get away with 1/2 inch on 6 meters, 10 meters or HF frequencies.

Heliax is a corrugated jacket feedline. Some early versions of it used a spiral corrugation pattern instead of the annular they use now. They changed the design after they discovered that rain and melted ice water was wicking down the spiral corrugation of the feedline and into the lower connectors. Another reason to avoid the spiral cable is that the connectors are no longer being made... all that is available is what's already out there.

Do not confuse the spiral feedline mentioned above with the Heliax that has a spiral plastic insulator between the center and outer conductors. I'm referring to the feedline whose outer jacket has a spiral pattern.

Waterproofing all of the feedline junctions outside the repeater site building is also VERY important. As I said above, be a very annoying, nagging perfectionist about connector installation and waterproofing. Waterproofing IS problematic on the "Y" junctions (also called "Y splices") in a multi-dipole phasing harnesses. BE CAREFUL with any RTV or "bathtub caulk" like compounds since many are based on acetic acid, and give off acid fumes as they cure. This IS the voice of experience with some Permatex corporation products.
DO NOT SCRIMP on the waterproofing of any connector outside the building. I repeat, DO NOT SCRIMP on the waterproofing. Yes, I'm being redundant. You absolutely have to keep the moisture out of any feedline or harness junctions. And note that the waterproofing is even more important in costal areas that have salt in the air.

Remember that Heliax is a hollow cable and uses a hollow center conductor. Imagine what happens if water migrates inside and freezes. If the water is inside the center conductor it will burst there. If it is between the center and outside conductors it will crush the center conductor, or burst the outer conductor, or both. Any of the above will destroy the cable. Yes, I've seen an ice ruptured Heliax cable... it was at the low point in the drip loop between the down-the-tower run and the building entry panel.

Moisture problems can be difficult to diagnose since common antenna test equipment (like those made by AEA or MFJ) use very low power (microwatts to milliwatts), and many moisture problems show up only with the full transmit power from the repeater transmitter. I saw one situation where the repeater just would not work right, and finally someone suggested that they check the feedline. They disconnected the jumper from the duplexer to the feedline and plugged the feedline end into a Bird Termaline. At that point things settled down and the repeater started to work properly. However the line had a SWR that was a little high, but usable. Damaged line has losses in both directions and the return loss can mask the damage. Someone decided to remove the Heliax connector on the bottom end of the feedline, and about a quart (a liter) of water poured out (it was the Heliax with the spiral insulator inside). The Heliax connector at the top of the tower had not been fitted properly... Everyone knows that green copper is corroded, but not everyone knows that green copper can act like a diode... and diodes create noise and grunge. One hint for you: if the copper is blackened you probably have water damage.

Before they were done they had to replace the antenna (that was for a different reason), the 9913 coax jumper between the feedline and the antenna ("But it came with the antenna, it has to be the right stuff"), and about 200 feet of Heliax (corroded internally). Not cheap, but necessary. And the second time they used someone who knew how to properly attach the Heliax connectors to the cable, and how to waterproof a feedline-to-antenna-jumper connection.

Superflex is an excellent product, but there's only so much flexing it'll do before it fatigue cracks or breaks. It's wonderful as jumpers between fixed objects but it's not intended to be as flexible as a test lead. And it's all too easy to "nick" the center conductor on it when installing the connectors. And a crack inside a jumper can cause real problems - just like a crack inside a collinear antenna. Read this article Help!! I have a crackling noise in my repeater.

From an email from Jeff WN3A:

Decibel attaches their harnesses to the mast using a fiberglass filament tape for strength, topped with an aluminum tape for UV protection. At one time, Decibel sold these tapes as replacement parts, but Andrew (since buying Decibel) no longer sells them (at least my distributor couldn't find them in the current price sheet). I had to re-research them last week to order more since I ran out. For those that are interested:
  • 3M 890 - Black polyester-backed glass filament tape. The original Decibel supplied part was 12mm width (0.47 inch), which seems hard to find. I was able to order 18mm (0.7 inch) width in small quantity from RS Hughes (www.rshughes.com). No distributor I could find would sell small quantities of the 12mm width, only by the case.
  • 3M 425 - Aluminum foil tape, 4.6 mil, UV/outdoor rated. Available from U-Line company (www.uline.com) in various widths (3/4" is what Decibel used).

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.



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