Antenna Cable Impedance-What's a Good Number?

[continuouswave]

Originally Posted by Wotname

Are you sorry you asked

I guess if it is not an anchor, then the thread is not supposed to go more than three replies of two sentences each. OK, I am learning what the animals in the zoo like to eat.

Next time I'll throw in more sidebars about warm beer.

[Wotname]

Originally Posted by continuouswave

I guess if it is not an anchor, then the thread is not supposed to go more than three replies of two sentences each. OK, I am learning what the animals in the zoo like to eat.

Next time I'll throw in more sidebars about warm beer.

However, I'm sure many of the other viewers learn heaps from the more elaborate posts such as the one in this thread; but I can't help wondering if the OP knew or wanted to know just how much there was to a seemly simple thing as characteristic coax impedance and the simple DC ohmmeter .

Now about that warm beer...

[redsky49]

Originally Posted by Wotname

Are you sorry you asked

I can appreciate their enthusiasm, even if the discussion has become a bit esoteric for my needs.

On the other hand, I learned that I only need a bigger antenna to broadcast to Clavius Base and beyond...

[Wotname]

Originally Posted by redsky49

I can appreciate their enthusiasm, even if the discussion has become a bit esoteric for my needs.

On the other hand, I learned that I only need a bigger antenna to broadcast to Clavius Base and beyond...

In keeping with the theme here, we now know that you only need to find another 35dB for Clavius Base; beyond will require more .

I haven't run the numbers yet but you could start small, say a quad stacked 15 element yargi. Failing that, move onto a large(ish) parabolic dish and circular
polarisation. You will need a bigger foredeck and some tracking apparatus to maintain line of sight for a suitable period to ensure time for a decent conversation.

Let us know how it works out

[continuouswave]

Originally Posted by Wotname

...I haven't run the numbers yet but you could start small, say a quad stacked 15 element yargi. Failing that, move onto a large(ish) parabolic dish and circular
polarisation....

Pardon my wandering off topic, but the mention of large parabolic dish brought to mind an old anecdote, which, if I may, I will relate:

Some time in the past history of orbiting satellites for communication of amateur radio (OSCAR), built by radio amateurs and carried into orbit by who knows what sort of connections or influence with NASA or the Air Force, I recall a situation in which an accident in the management or supervision of one of those satellites resulted in a hopeless situation that disabled the satellite.

As I recall, an error was made and a transmitter on the satellite was accidentally turned on that should not have been, resulting in the desensitization of the 432-MHz command receiver that could be used to fix this error. The amateur radio stations that were controlling the satellite could not generate a signal up to it that was strong enough to overcome the desensitization of the command receiver from the other local transmitter. The situation was not looking good.

However, again using who knows what sorts of connections and influence, an arrangement was made to borrow a very large parabolic antenna and to connect to it the appropriate 432-MHz amateur radio transmitter. The big parabolic antenna--owned either by some big university in California or perhaps by the Air Force, I don't recall precisely--was then configured to track the OSCAR. The full amateur legal limit of 1-kW (or perhaps more) was pumped into the big parabolic, resulting in an ERP of many megawatts.

This produced enough signal at the satellite's command receiver to overcome the desensitization, and the transmitter causing the problem was switched off.

By the way, radio amateurs have, for some time, been using the moon as a reflector, and communicating bouncing signals off it back to earth. This does require a rather substantial antenna array, but, lately, using new digital signal methods which permit much weaker signals, it apparently has become possible to accomplish with more modest antennas.

OK, enough of my sidebar. Back to warm beer.

I have a story for that, too. Living in SE Michigan many folks around here are working in the automobile business. I had a good boating friend and neighbor who worked for General Motors as a designer. He threw a holiday party one December, inviting a lot of buddies from work. At that party a lot of the banter was, naturally, related to automobiles. It was there that I heard this joke:

Q: Why do the English drink warm beer?

A: Lucas Electrics

[Billylll]

1st 85 feet is a pretty long run for VHF. What type of cable are you using Times Microwave LMR400 or LMR600? Return loss using an Anritsu Site Master would be your best bet to test the cable and connectors. The cable is tested with an open/ short and 50 ohm non reactive load. The distance is measured using the open/ short the impedance is measured using the load. Using an ohm meter is almost worthless when it comes to checking a cable for VHF unless you disconnect it from the antenna. Then you want little to no measurable resistance from the center to center and the shield to shield. You want to measure an open between the center pin and the shield. Many VHF marine antennas are a DC short to ground to protect equipment from static discharges and to some degree lightning. At 85 feet I really hope you are using LMR600 or Andrew FSJ-50 superflex. You will need to transition to jumpers with the correct connectors at both ends. Make sure in case it wasn't already mentioned that the cable you are using has a 50 ohm impedance. 75 ohm is for Satellite, CATV or CCTV cameras never use it for VHF radios.
A good test is to tune into the various weather stations run by NOAA you typically should be able to hear a NOAA VHF weather station out to at least 13 miles.
Bill

[redsky49]

I am using 50 ohm LMR-400 cable, but don't think that 85' is unusual/excessive. Top of mast to bottom is 60', a couple of extra feet at the bottom to make a disconnect coupling if I take the mast out again, about 18' +/- to the radio at the nav station, and a foot or two extra.

I am sure that sailors with taller masts frequently have over a hundred feet (or more) of cable so the length doesn't worry me. The prior cable was RG-8U and performance was very good. Now with a better antenna, better cabling, and the elimination of a prior in-line coupling, performance can only be improved.

I think that practical testing will be the best alternative for me. I will see how far I can transmit and receive once the mast is back in (Tuesday, if weather cooperates).

I get the impression that most of the responses I have received are from radio enthusiasts, and I don't have access to the equipment that geeks (and I mean that in the nicest way) typically have. As they say, the proof is in the pudding, and I intend to sample that next week.

Thanks again!

[Billylll]

You are loosing almost 1.5db that's a fair amount of loss not terrible, however the added height you have above average terrain is excellent. Your VHF should work very well with that setup.
From Times Microwave site:

Product: LMR400
Frequency (MHz): 162
Run Length (Feet):85


PRODUCT PERFORMANCE PARAMETERS

Attenuation: 1.6 db/100ft 5.2 db/100mtr
Average Power: 1.41 KW
Cable Vg: 85 %
Nominal Td: 1.2 nSec/ft 3.92 nSec/mtr
Capacitance: 23.9 pF/ft 78.4 pF/mtr
Typical Connector Loss: 0.03 dB/pair

CABLE ASSEMBLY PERFORMANCE

Cable Run Attenuation: 1.4 dB
Total Cable Assembly Loss: 1.5 dB
Cable Run Efficiency: 73.1 %
Cable Run Time Delay: 101.6 nSec

[redsky49]

Wow. Glad I didn't go with RG-213, or with RG-8U!