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Author Topic: New QTH and new tower - Coax/Conduit Question  (Read 2598 times)

Posts: 4

« on: April 25, 2011, 03:23:26 PM »

I'm trying to sell my house and move into the country where I can build a big tower and no covenants can tell me to take it down. 

I'm researching coax feed line and wondering what is the best (cost vs. performance).  I do know that I should put in the best I can afford since this is a key piece of my station.  I work in the low microwave bands at work and frequently use LMR-400 for short runs and Heliax for long runs.  With experience in both I got to thinking, Could I simulate Heliax or other hard line low loss characteristics by placing a run of LMR-400 in a 1/2" EMT conduit?

I will be trenching around my tower for ground lines and had planned on burying my feed lines in 1.5" PVC conduit from the shack to the base of the tower anyway.  I'm curious to know if I used LMR-400 inserted into 1/2" EMT, painted to protect against rust, and then inserted into 1.5" PVC buried a foot below grade, would the 400 and conduit combo decrease the loss?

I don't have any modeling software or much of a basis to guess about this.  I hope to find someone that has tried this or someone smart enough to know if this is worth experimenting with.

I would guess that the solid shielding of the conduit would block EMI on the outside and also prevent leakage from leaving the conduit, but any interactions between the coax shield and the conduit surface is where I get lost.

Thanks for your time,

N5 Eric Martin X
aka KE5SFY

Posts: 21758

« Reply #1 on: April 25, 2011, 03:33:30 PM »

Not at all (about the conduit decreasing the loss).

Almost all the loss in coaxial cable used at common frequencies comes from the surface resistance of the center conductor, and almost nothing else.  The outer conductor, having much more surface area, contributes very little, and dielectric losses are negligible until you get into the SHF region.

Also, no matter what you add to the outside of coax, all the RF current is conducted by the innermost surface of the outer conductor; so, the more conductive that is and the thicker the skin depth (based on frequency), the less loss it will have.  Once you get outside that conductor, it doesn't matter what you add.

Hardline, Heliax, etc. get their low loss by using larger diameter center conductors which in turn lead to larger diameter outer conductors as well, to maintain the desired characteristic impedance.

Posts: 4

« Reply #2 on: April 25, 2011, 03:43:06 PM »

Thanks for the quick response.  Not what I wanted to hear when Heliax is $3 a foot, but you saved me the time and trouble of experimenting.



Posts: 2415

« Reply #3 on: April 25, 2011, 10:16:04 PM »

The good news is that good USED Heliax is available cheap at most all swapfests, And if it can be shipped (The smaller 1/2"  LDF4-50A)  Ebay sometimes has good deals on it.
I suggest larger size PVC conduit than 1 1/2 inch.     100 foot rolls of the corrugated plastic drainage tubing is sold at farm type stores for just over 20 bucks a hundred foot roll in 3 inch, 4 inch is only slightly higher.

When you bury the conduit, Be sure to also put a bare copper ground wire of at least #6  outside the conduit to bond your ground systems together. (Flat copper strap (Copper roof flashing) Or soft copper tube also work well)

The length and type of coax used depends on the frequency vs the overall length.  VHF/UHF require good low loss coax that be kept as short as possible.  The low bands on HF, Not nearly so important.

If cost is a real factor, Don't forget that surplus cable TV trunk "hardline" is usually available at real close to free.
A bag of donuts to a construction crew will usually get you a few "spool ends" of a hundred or more feet........

Dont forget to ask for a few extra CATV "PIN" type connectors to go with it. They adapt directly to PL 259 type connectors!

Posts: 378

« Reply #4 on: April 26, 2011, 10:47:17 AM »

You can get NEW Andrews Heliax and connectors on E-Bay all the time.  Most of these sales come from roll ends that were left over from projects.  I've gotten 1/2" hardline for as low as $.75 a foot and connectors in the $7.99 range by being patient and waiting for a good deal.  Most of the rolls of hardline will be in the 100 foot range.  Which, for my 65 foot tower was perfect.

Posts: 2264

« Reply #5 on: April 26, 2011, 05:34:29 PM »

I've replaced just about all my transmission lines with LDF4-50 purchased at hamfests and at Dayton for literally pennies on the dollar. Most of it along with rotor and control lines are run in buried 3" electrical conduit which has several down facing Tee's along their run to insure proper drainage.  A buried length of pipe will breathe and will fill with condensate if not properly drained.  I leave no pull lines in the pipe, but prefer to blow a fresh line thru as needed by first blowing mason's line attached to a plastic bag "ghost" followed by the actual pull line.  A fresh line insures that there are no helical twists in the cable as may sometimes happen if a pull line is carried along with the run of cable.  To blow a line, I use the exhaust from a shop vac.  It will shoot it so fast thru the pipe, the line will literally burn your fingers as it's paying out.  Each end of the pipe is stuffed with coarse stainless steel wool to prevent insects and critters from making a home in it.

Posts: 680

« Reply #6 on: April 27, 2011, 08:14:12 AM »

Almost all the loss in coaxial cable used at common frequencies comes from the surface resistance of the center conductor, and almost nothing else.  The outer conductor, having much more surface area, contributes very little, and dielectric losses are negligible until you get into the SHF region.

The ratio of losses between inner and outer is approximately the ratio of inner to outer diameter, or about 1:3.3 for solid PE dielectric 50 ohm cable. I don't know that this is "almost all", but certainly, I'd go for 75% of the loss is in the center conductor.  For foam or air dielectric, the ratio is smaller (LMR400 is 1:2.46), and for the same outer diameter, the inner conductor is bigger, so the loss is lower, but the shield starts to be a bigger fraction of the total loss.

I ran across an interesting anomaly in the whole coax loss thing the other day: A cable might be really good for microwave, but not so hot for HF.. Some cables for microwave purposes have a very thin layer of silver or copper on a stainless steel core.  At microwave frequencies, the skin depth is much less than the cladding thickness, but at HF, that's not the case.  (LMR400, for instance, is copper over aluminum).  The really big coax actually has a tubing center conductor.  I'm pretty sure the inner tube wall is thicker than the skin depth, even at low HF, (copper is 33 microns, 0.0012" @ 4MHz), but it would be something to check. As a rule of thumb, you want the conductor thickness to be 5 times the skin depth.

For a small diameter conductor, <10 skin depths in diameter,  the AC resistance is even higher.. for very small coax 'WIK is probably closer to the truth when he says "almost all of the loss is in the center conductor", but even something as small as  RG-8X or RG-58 has a center conductor around 1mm diameter, which is like 30 skin depths at 4 MHz.

I've been looking into this in connection with inexpensive coax (e.g. RG-6) intended for Cable TV kinds of applications, a lot of which has copper over steel core center conductors. 
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