Coax length for TX / adding feet

[ChetsJug]

Okay so I understand that coax length is important. Like 18 feet of RG-58 is 50 oms.

So what about getting that 18' of coax to the mast and up to the antenna if it is a total of 40' or so? Does RG8 go by the same rule? I'm lost on oms. How do I add length to get to the antenna?

I'm doing two applications. One is a C.B. base antenna that will be on an old flagpole for a mast. About 20' tall (above cement surface). It will be just off the pool fence, about 25' from the corner of the house. Then 10-15' inside the house. About 60' in all (could be more).

The other will be a VHF antenna on the top of the swamp cooler. It's MURS so I want to get the SWR as best I can having only 2 (5) watts to work with. I have the large MasterCool type. A nice ground plane area for a magnet mount I have. (I ran a Kraco center load Magnet Mount up there for 15 years lol). So the coax is 15 feet with a PL-259. If I make an extension lead to get it another 10-15' to the base unit and put a BNC on it, I can plug it onto the base. What about SWR on VHF? I've never done that before. How does oms affect VHF tuning?

You can tell I'm a coax twit, help me please

 

[WA0CBW]

In simple terms the length of coax has nothing to do with its impedance (50 ohms). The size of the coax (RG58; RG8; RG8X) determines how much the coax attenuates the signal per 100 feet. In general the larger the coax the less attenuation per 100 feet. RG8 would have less attenuation than RG58. Also the higher in frequency the more attenuation per 100 feet. The same piece of coax will have more attenuation at MURS frequencies (150 MHz) than at CB frequencies (27 MHz). It is a good idea to have as few as possible connectors/adapters in the coax as each adapter/connector adds to the loss of signal on both transmit and receive.

SWR is the match between the impedance of the coax (50 Ohms) and the impedance of the antenna (suppose to be 50 ohms). Impedance cannot be measured with an ohm meter. The impedance of an antenna depends on the length of the antenna and the method used to match the antenna to the 50 ohm coax. Different lengths of antenna (1/4; 1/2 and 5/8 wavelength) have different impedances. Usually SWR is much more important for transmitting than receiving.
BB

 

[jonwienke]

There is only one rule regarding coax (or any transmission line) length: make it as short as possible to go from transmitter to antenna. You should not try to lower SWR by changing coax length, you minimize SWR by tuning the antenna properly (usually by adjusting its length).

You will need an SWR meter that covers CB and VHF. The tuning procedure is the same regardless of frequency.

 

[Oldme]

There are many myths about coax.
What you really need is from the radio to the antenna.
NO more for normal antennas.

The only antennas that call for any certain length of coax
are some of the NO-GROUND PLANE antennas.
They will specify that in their instructions.
Shakesphere has several with their marine antennas
as does Firestick.

Otherwise shorter is better.
Use the best quality with the best shielding you can afford.

 

[ChetsJug]

Starting to see the light

Great

So then using a barrel connector (pl258?) and a second length of coax to the mag mount on the swamp cooler was not all that damaging. Hey, I was 16 and it was my first CB. A friend gave me his Kraco mag mount. I put it on top of a metal fence post. I couldnt get out. Dad, an ex hammer said it needed a ground plane. So I put it on the swamper and got out 7 or 8 miles (1976). A few years later I was told coax had to be 18' because it was 50 oms, bla bla bla. (Sorry, I'm being redundant.)

I have taken another look at the outside of the house. I would like to run a pole mast up the side of my bedroom and that would hide the antenna in the center of an elm tree canopy. Great for not having neighbors see it and relentlessly knocking on my door over TVI. Trouble is there is a phone pole in that tree as well. Do you think the neighbor would get any TVI over Fios or their phone? I've heard that radio can mess with the digital contraptions in line.

That's it in a nut shell. My distance was to put the antennas just so in the yard as to hide them from front street neighbors and stand far enough away from that pole in the tree (narrow side yard).

i'll get 'er all up, hook up an SWR meter, We'll go from there.

That'll take a month as I drive coast to coast. I'll get back soon.. Thanks

 

[Oldme]

Sounds like you have a plan coming together.

Have a Safe trip.

 

[WA0CBW]

The reason for the 18 ft of coax is that 18 feet is about 1/2 wavelength at CB frequencies. When you put your SWR meter 1/2 wavelength (or multiples of 1/2 wavelength) away from the antenna (load) the coax will reflect whatever impedance it sees at the antenna. This gives you a more accurate indication of the match between the coax and the antenna when the SWR meter is at the radio end of the coax instead of at the antenna where it would measure the true match between the coax and antenna.

 

[Oldme]

Searches will give many answer. That is why myths persist.
If you do not have Common Mode Current problems
*** or taking away coax will not change your SWR.

If you have a bad match / poorly tuned antenna,
coax length will show a difference. 50 Ohm cable
to match a 50 system.
Do not make it any longer than necessary.
See #2
Coax information

The Ultimate Guide to 11 Meter CB Antennas

With proper matches the SWR should be the same at
10' or 100. Now you will have more transmission line
loss with the longer run.
Coax Calculator

Interesting info:
http://www.ea1uro.com/pdf/Practical AntennaHandbook vol 4.pdf

Have fun.

 

[ChetsJug]

Thanks OldMe and everyone else. It took some shaking the cobwebs out but I remembered my dad had a famous (infamous) green big antenna book. Known to many as the Antenna Bible in the 60's n 70's i've heard. I think my uncle has it.

Anyway, I have decided to mount all antennas on the house; not the yard fence etc. This will keep coaxial lengths at the shortest and I'm to old and fat to go climbing towers

I watched a few YouTubes to find out how to make a 102 whip ground plane. I love creating things and proving you don't have to buy expensive toys. Every time I screw a professional mechanic out of some money because I found a way to fix my own car, the better I feel. So after deciding what I wanted to make, I was looking for some mounting stuff on ebay and there it was...

So much for DIY home made antennas. I broke down and purchased a Sirio 827! I am sticking to the magnet mount VHF antenna on the swamp cooler for the MURS though lol. It's 2 watts and I doubt I'm getting any DX out of it this century.

Okay so I'm 52 and need to save my energy for my '23 T-Bucket and my '64 Chevy pickup... And after my dad passed away, I realized his 1968 Datsun Roadster (SPL311) was sitting under the patio roof and was now mine by default (heir apparent). Oh, and the 1977 Harvest Class C motor home... Don't forget the 20' Commander jet boat. I really love car mechanik's and have no desire to make radio stuff. I'm chickening out before I get started lol

I will look all this stuff up though and make sure everything is efficient and matched.

Tanks again

 

[Oldme]

Looks like you have your plan and i hope it all comes together.

Believe me, I understand the deal on climbing.
I turned 67 last week and I think tall ladders are behind me now.
Today I had my eyes checked and there went the hobby money.
Life.

Hope you enjoy your setup.