Should I use 50 or 75 ohm coax?

[Bill2k]

I'm trying to iron out the last few details for a yagi antenna built similar to the dimensions from K7MEM's website: Martin E. Meserve - K7MEM - VHF/UHF Yagi Antenna Design

From what I've read, this antenna has an impedance closer to 75 ohms. My question is... would it be more efficient to use 50 or 75 ohm coax to connect the antenna to my scanner which has 50 ohm impedance?

BTW, The antenna have a center frequency of around 868MHz.

 

[davidgcet]

really wont matter much for RX only, there may be a little less of a match using 75 than 50 but not enough to matter unless you trying to grab a signal a LONG way away.

 

[Bill2k]

I'm going to be using the yagi to strengthen the signal from a city which is about 20 miles away. I can pick the city up with a radio shack rubber ducky 800MHz antenna, but the voice volume is low and the noise volume is high. I can hear people talking but its hard to understand them unless I really concentrate on the audio. What I'm looking to do is strengthen the signal to hopefully make the voice easier to hear. I wasn't sure if it was better to use coax matched to the receiver or the antenna to prevent loss or noise.

 

[davidgcet]

it is best to match the whole thing, RX/coax/antenna. but if you are picking them up on a duckie now then you should not have any issue with a yagi. don't go over board on gain, too much gain and you increase the noise floor and may have worse RX than you get now.

 

[Bill2k]

The yagi has around 12dbi gain, I hope its not to much gain, I guess we'll find out when I build it.

 

[mass-man]

Yea, you can always add an attenuator if necessary!!!!

 

[exkalibur]

Yeah, you won't have an issue with 75 Ohm.

Here's the thing. 50 Ohm coax sacrafices a little bit of loss for the ability to handle higher RF power. 75 Ohm sacrafices a little bit of RF power handling for slightly less loss.

I don't have the math in front of me, but I've heard that whatever loss you loose in impedance mis-match using 75 instead of 50, you'll make up for with the slightly less loss in the coax itself.

End result? There isn't going to be a noticable difference at all. I've used RG-11 and 75 ohm hardline in the past, and never noticed any issues at all either power, performance or SWR wise.

 

[W2NJS]

The theoretical impedance of a dipole, not a folded dipole, is 75 ohms. For receive-only situations it does not make a lot of difference which type cable you use. I'd concentrate on getting high-quality cable of whatever kind, making good connections, and avoiding the use of plug adapters.

 

[mpddigital]

Not to mention that good 75 ohm coax is less expensive to produce and costs less! We started carrying LMR-400-75 for scanner users for this purpose but Good RG-11 coax will work as well for your purposes. Look for Commscope, Times, Coleman or Alpha coax and you will be set for many years.

On the Db Gain issue, we've fought this with customers for years with customers buying cheap amps and antennas wondering why their signal isn't much better. The analogy I use is turning up the music LOUDER without filtering our the crackle and hiss doesn't do much for understanding the song. Signal to noise ratio plays a big part and getting the right antenna system for your application is important to get the reception you want. Sounds like you are on the right track.

 

[K9WG]

I have been using 75ohm satellite coax for several years on both TX and RX without any problems. Good quality satellite coax has less loss and better shielding at UHF then cheap 50ohm coax.

 

[mpddigital]

For TX 75 ohm works fine for work in the HF and VHF bands but tends to attenuate far greater than 50 ohm as you go higher. Above 300MHz it works best for matching and co-phasing but 50 ohm will work much better all around. Assuming you don't buy cheap...

 

[Bill2k]

Wow! Thanks for all the suggestions everyone. I think I'll pick up some low loss 75 ohm coax and get this thing underway.

 

[redshift]

I was going to ask the same question -- glad I found this thread. I have 3 extra RG-6QS feedlines I originally ran from the basement to the attic for my satellite dish that are now sitting there taking up space. I was hoping I could reuse them as feedlines for a VHF base antenna (2m ham, max 50W TX) and possibly a wideband receive-only antenna for the scanner.

From reading this thread, it doesn't sound like it will be a big deal, which is nice because I don't have room in the pipe for anything thicker (ran a 2" conduit when the house was built - should have found a place for a 4").

 

[mpddigital]

With your scanner the RG-6 will work well. For RX TX on VHF the RG-6 will work but it is not the best solution for all around use. In most cases you will get better service with 50 ohm designed for radio use. If size is an issue go with an Alpha, Commscope or Coleman RG-8x or Times 240. The 240 has extremely low loss at VHF freqs.

 

[sfreiman]

Am I reading these posts correctly that 75 Ohm is fine to use as long as it is for receive only? Does this apply to shortwave antenna lead ins as well? I'm moving to a house that has a ton of 75 Ohm lead ins so it would be great if I could utilize those without having to rewire.

 

[mpddigital]

Give it a shot. I know little about shortwave reception and antennas but it would seem that 75 ohm would work fine at the freqs you are pulling in.

 

[Placid1]

Just make sure whatever type connectors you intend to use match physical size wise.
I know a 75ohm and a 50ohm N-type connector are supposed to be a different physical size (center pin). A mismatch can cause a poor connection or a bent center pin.

75ohm top. 50 ohm bottom.

 

[Nap]

I'd recommend the article here:

Dipole antenna - Wikipedia, the free encyclopedia

If you want to do it properly, you have to use a balun.

Otherwise:

- if you use 50 Ohms cable, you'll have an impedance mismatch at the antenna end
- if you use 75 Ohms cable, you'll have an impedance mismatch at the scanner end

 

[W6KRU]

- if you use 75 Ohms cable, you'll have an impedance mismatch at the scanner end

At what frequency is the scanner input actually 50 ohms?

 

[Nap]

At what frequency is the scanner input actually 50 ohms?

Which scanner?