What it the optimal number of wave lengths of RG-8X coax to connect my Icom IC V80 HT to a 50 ohm Jetstream JTBM270 Yagi (3 elements 2m/5 elements 70cm)? Odd or even number thereof?
Will running the coax through a steel mast inside an abandoned cast iron sewer vent pipe affect my signal?
Antenna has type N connector and HT has BNC.
If the antenna is operating correctly, the length of the feedline and the material around the feedline (vent pipe, etc) are irrelevant. This is because coax is designed to contain the electric field in the dielectric between the center conductor and the shield. With no electric field outside of it, what happens outside the cable is irrelevant. This is why coax has supplanted twin-lead in most applications: you can attach coax directly to a metal tower, bury it in the ground, or feed it through a wall with no effect on the signal.
However, we must ensure current balance at the feedpoint for this to be true. Current balance means that the current into one half of the antenna equals exactly the current out of the other half of the antenna at all times. The law of charge conservation ultimately requires that all the current goes somewhere, and if we aren't keeping the currents on the antenna elements equal and opposite, then the feedline will make up the difference.
To ensure current balance in situations like these we use a balun. This is already covered in detail for dipoles in Using a balun with a resonant dipole. The driven element of your Yagi is a resonant dipole, so this applies to your antenna just the same way. Note though that an insufficient balun in a Yagi is especially bad, because it makes the feedline part of the radiating antenna structure. You want a Yagi because it's directional, but the feedline as an antenna is anything but directional. Common-mode currents will destroy the performance of your Yagi.
I did some brief research on your particular antenna, and I couldn't find any information on whether it has a balun or not. So, you might be interested in How to detect common-mode currents or “RF in the shack”?
The photo on the manufacturer's web site suggest that the beam uses a gamma-match arrangement. The cable shield grounds to the boom, whilst the central conductor is routed on a bar parallel to the driven element and connects to the latter at some distance out from the centre via a series capacitance. Matching is carried out by adjusting the distance out from the centre at which the contact to the driven element is made and the series capacitance value (which resonates against the inductance of the feed-point).
In my experience this form of connection is well balanced, and a good match can be obtained with a little effort. It is best to start with this.
There is no "optimum" length of cable for a well-matched antenna. As you are operating at UHF and VHF, the only optimum length is zero - more cable simply gives more loss...
Running your cable inside the mast and then inside a pipe will actually help by adding shielding (and won't hurt at all) but you may want to prevent any imbalance currents running on the steel or (worse) on the iron (rusty joints may act as rectifiers and give harmonics!).
Although this is probably overkill, it may be worthwhile just as a safety measure to add either a coil of coaxial cable just after the feed-point or some (clip-on?) ferrites around the cable. The coil might be (say) 6 or 8 turns of the coax wound on a 3-inch to 4-inch plastic pipe segment. Either scheme adds some inductive impedance to prevent any accidental outer-braid current from flowing.
You might be able to test whether or not you have a problem by trying to detect any "squint" in the antenna's beam pattern (which is often caused by the unwanted radiation coming from an "outer-braid" current). Mount the antenna clear of obstructions; then, using a beacon or similar constant and "good strength" distant station, swing the antenna and look for two equally sharp "nulls" in signal which should occur at equal angles from the direction of maximum signal strength. Both nulls should be similar: the same loss of signal over thee same angular range. The maximum signal should occur when the boom points at the station (unless you have hills and buildings around!). Then make a temporary choke in the coax - coil up a few turns (say 6) at 4 to 6 inch diameter just below the feed point and hold them in place with plastic (!) cable ties. Repeat the beam pattern test. If the direction of maximum signal changes, and the two nulls become more similar and more symmetrical about the direction of maximum signal, then you had a problem and you have just fixed it. If there is little or no change (or things deteriorate), you probably did not have a problem in the first place.
One last (probably obvious) point: I am assuming that the antenna will be used for horizontal polarisation. The antenna balance would be seriously compromised if it were mounted fro vertical polarisation on a steel mast!
Good luck experimenting...
