Wednesday, September 23, 2015

For Future Consideration: Transmission Line

This is the second in what may become a series of articles about my preparation for a larger antenna farm, perhaps as soon as 2016. The first article was about my acquisition of a 2-element 40 meter yagi (XM240). This one is about transmission lines. That is, low loss coaxial cable.

The problem

In most HF stations the quality of the coax from shack to antennas is often more about robustness, power handling and weatherproofing. Loss on the HF bands for runs under 100' (30 meters) is usually low for RG-213 (or equivalent) coax, and it will handle a kilowatt if the SWRis modest. For example, a 50' (15 meters) tall tower and tri-bander with 75' of RG-213 has a matched loss of less than -0.9 db at 28 MHz. With an SWR of 2 the total loss can be as low as -1 db. Loss progressively declines on lower bands.

In a large station transmission line loss is often a concern. Consider a 100' (30 meters) tower that (for peace of mind) is a similar distance from the house. With additional cable at the top of the tower and into the shack we are looking at 250' (75 meters) of coax. At 28 MHz the matched loss of RG-213 is now -2.9 db and at least -3.3 db with an SWR of 2. Upgrade to a 150' (45 meters) tower under the same scenario and losses increase to -4.0 and -4.4 db, for an SWR of 1 and 2, respectively.

Of course the situation is less dire at lower frequencies. RG-213 can prove adequate at 7 MHz and lower even for these long runs. But for the hams that build these large stations every decibel counts, since it can make a difference in contest results. This is due to impacts on transmit (getting heard) and receive (copying the weak stations attracted by the big signal). There are other matters to be considered, such as whether the coax can be buried, moisture wicking by a braided outer conductor, UV resistance and more. This is why many hams eschew RG-213 and similar coax, since quality is often questionable. They instead use brand name cables in the same size such as Times Microwave LMR-400.

Better coax can be had, if one is willing to buy new or one is able to scrounge. The former takes money and the latter takes time and effort. I favour the latter, if at all possible. Time is on my side since I will not need more transmission line until at least 2016.

Andrew Heliax

When I built my modest station in 1985 I was fortunate to purchase new hardline at a bargain price. This was 400' of Andrew Heliax LDF4-50A, ½" 50 Ω coax. Even at the low per-foot price I paid it was still a goodly sum of money. I was fortunate to acquire new connectors at wholesale price through a friend in the business since connectors can be very expensive. Divided into 3 lengths of 130' this was still a substantial expense for a set of transmission lines from the shack to the top of the tower.
400' of LDF4-50A hibernating in the garage.

The runs were 130' (40 meters) because the tower was 19 meters high and a similar distance from the tower to the shack. Cables were buried for part of the distance. The antennas atop that tower were yagis for 20-15-10 (Hy-Gain TH6DXX), 6 (Cushcraft A50-6) and 2 (Cushcraft 32-19). RG-213 was used for 40 and 80, which I purchased new in bulk, on the spool. It is these same 130' lengths of RG-213 I am currently using in my station today, after having been in storage for over 20 years. There is still 100' or more on that original spool. The rest went to friends who participating in the bulk purchase.

The LDF4-50A has also been in storage. Heliax is very robust, suitable for direct burial, reasonably immune to moisture wicking, and will support its own weight when hanging from the tower top and (if the tales I've heard are true) survive being run over by motor vehicles. Loss is substantially better than RG-213 despite not being much thicker. At 28 MHz the matched loss is -0.35 db per 100' (30 meters). For the 150' tower mentioned above, the loss over 350' is -1.25 db. That isn't bad but, as we'll see, we can do better.

Buying hardline

When buying Heliax one decision is whether to buy new or used. As with anything purchased used it is a good idea to test first, if at all possible. In the case of coax you want to do a frequency sweep of unloaded cable to ensure it is close to spec for loss and impedance. I've heard of hams who carry a VNA to flea markets for this very purpose. Some old or new but rejected cable may show impedance anomalies above 1 GHz, but be perfectly good at HF, VHF and even UHF.

Used cable often has the advantage of having one or even both connectors attached. With the high price of connectors this can be an advantage. With care the connectors can be removed and reused, and the cable cut (if necessary) to the needed length. Shorter lengths can be connected to make a longer run, provided that splices are not buried. Splices must be waterproofed and use the correct connectors.

There is a commonly-heard myth that connectors are lossy and must be avoided for best performance. This isn't true at HF, and not true at VHF and UHF is constant impedance connectors are used (e.g. N). The trade-off is the whether the price of the hardline is low enough to justify buying connectors. They are often hard to find the used market, and are rarely inexpensive.

As we should expect bigger coax has a bigger price. This is where scrounging comes in particularly handy. However, the situation is far from hopeless. Back in 2007 Andrew stopped producing LDF cable, replacing it with the improved AVA line of products. More LDF began finding its way onto the surplus market at attractive prices while the commercial world made the switch to AVA.

If you do scrounge you will find that surplus new LDF4-50A and even LDF5-50A often costs little more than new LMR-400. But, as I said, scrounging takes time. When the opportunity appears you have to act, since you don't know when or if another deal will come your way. Be especially acquisitive of connectors should they be offered at a good price. If you don't need them you can always sell the surplus to your friends. You'll put a smile on their faces, and probably have a ready crew for your next tower project!

Bigger is better

Back in 1985 there were hams who found it funny that I'd run Heliax on HF. For short runs, they had a point. However, the specs of the no-name coax most of them used was not to be trusted. At least I knew what performance I was getting, and that the coax would hold those specs for a very long time even when exposed to the weather or buried.

Hardline specs in TLW do not exactly match Heliax
But that's for short runs. For longer runs at competitive HF stations (or short runs on VHF and UHF) bigger cable than LDF4-50A is desirable. The most common choice is the next step up in size: LDF5-50A. This coax is ⅞" in diameter, with a proportional improvement in loss and power handling. At 28 MHz a 100' length has a matched loss of -0.2 db, or half the loss of ½" Heliax as expressed in decibels. Loss over exceptionally long runs can be better than -1 db up through 10 meters, and even 6 meters.

Specs for LDF and AVA are easily found on the internet. You can also use N6BV's TLW application to approximate loss performance for Heliax. LDF7 and AVA are not in the menu so you'd have to make those a user-defined cable type. TLW is packaged with recent editions of the ARRL Antenna Book.

Going all in

This series of articles titled "For Future Consideration" is about actions I've taken to prepare for my future station. The foregoing discussion is just a lead-in to the latest step I've taken. Sometimes when you least expect it the used market tosses exactly what you need into your path. You have only to pull out your wallet when opportunity knocks. This is what I've done.

Luckily my one-car garage is fairly deep, allowing room for storage. That space has been filled over the years with lots of stuff that I should have sold or thrown out years ago. This I have now done to make space for my newest arrivals.

In the photos at right you can see that I am planning for some long runs of low-loss transmission line to multiple antennas on tall towers. The 400' of LDF4 is dwarfed by the new rolls of Heliax piled up against it. Most of it is LDF5-50A, both new and used. Connectors are either attached to the used lengths or new in the box. These are a mix of N and 7-16 DIN connectors, and at least one UHF connector.

The new LDF5-50A is on the original wooden spool. It weighs a lot. Two of us were needed to roll it up the steep incline of my driveway. Since the photos were taken most of the separate rolls have been placed atop the spool to conserve space and to protect them from wintertime saltwater contamination.

There are also several short runs of cable (<10 meters each) that are suited for stacking harnesses and house entry. Or I can cannibalize them for the connectors.

As additional eye candy there is a roll of LDF7-50A, which is 1-⅝" diameter. The matched loss on this cable is virtually nil on HF (-0.1 db/100' at 28 MHz). Is this size of cable really necessary? Perhaps not on HF, unless the towers are 100 or more meters from the shack, but I do have some interest in returning to VHF DXing. Some contest super-stations use this product on the upper HF bands.

To give an idea of how large this cable is the adjacent selfie photo shows me holding a DIN connector for LDF7-50A next to the coax roll. Putting connectors on large Heliax requires some large tools and care but is not much more challenging than attaching connectors to more ordinary varieties of braided coax. There is no soldering to be done, and the large size eases handling.

Do you really need hardline?

Only you can decide. How much is 1 db worth to you? Or 3 db, the equivalent of stacked yagis? At HF the low loss of hardline is not needed for receive since reception is noise limited. The situation is quite different on VHF and UHF, and even 10 meters, unless you use a remotely switchable low-noise amplifier (LNA).

Transmit is another matter. Country regulations and award programs, including contest categories, almost always specify power at the transmitter. This is as true for QRP as it is for QRO, although the motivations may differ: protect what little QRP power you have, or emit the strongest, legal QRO signal. For everyday use it is often more convenient to use an amplifier to overcome transmission line loss. That is, if you are not already at the legal limit.

Low loss can give you the flexibility to locate your tower and antennas further away, to reduce noise pickup and interactions with buildings and other antennas, and perhaps allow use of a better area (a small hill?) for construction, maintenance and radial field. Although this wouldn't apply in a typical urban or suburban lot, many hams move to rural locations to fulfill their radio ambitions.

I did the arithmetic for what I want to achieve with my future antenna farm. Purchasing an abundance of Heliax was the right decision, for me.

Matched loss versus SWR loss

One final note on the limits of low-loss transmission line. This is as true for open-wire line as it is for coax, even if some books say otherwise. To give an example I'll draw your attention to an article I wrote some time ago, regarding the practical limit of solving antenna mismatches (high SWR) with a tuner in the shack.

In that article I discussed the challenge of attempting to use antennas for higher bands on 80 meters. My choices at the time were an inverted vee for 30 through 10 meters and a 40 meter delta loop. The SWR in both cases is exceptionally high due to the low radiation resistance. The feed point impedance at 3.6 MHz of the inverted vee was 3 - j1000 Ω, and of the delta loop was 0.5 - j80 Ω.

The matched loss of 130' of RG-213 is -0.5 db on 80 meters. For the inverted vee and delta loop on 80 meters the total losses are -25.6 and -13.8 db, respectively. For the same length of LDF5-50A the total losses would be -17.6 and -6.2, respectively. These are better but still far too large.

The lesson is that when using coax with a very low matched loss, correcting a high SWR with a tuner in the shack remains a poor choice. It is almost always better to correct the mismatch in the antenna design or with a matching network at the feed point. Tuners don't solve all ills, and neither does Heliax.

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