As ham radio operators, we’re always looking for a way to save a buck. I’ll frequently dig through a free bin of part, hoping to scavenge something for the next project. This scavenger mentality extends to coax feed-line. High quality coax cable can be expensive. That’s why we love a deal when can find it. The cable television companies go through miles of coax cable on a daily basis. Often a ham can pick up free pieces and reel ends for pennies on the dollar. Or just take a trip through your local home improvement store and you’ll see 100 foot spools of 75 Ohm RG-6 for a fraction of the cost of similar 50 Ohm RG-8U.
But are 75 Ohm cables really a good bet? Will it work for ham radio? Or am I setting myself up for aggravation? This article explores the good and bad of 75 Ohm coax cable.
Impedance mismatch of 75 Ohm Cable
RG-59 and RG-6 are 75 Ohm coaxial cables designed for cable television applications, they tend to have wide frequency response and low line loss. They also have an impedance of 75 ohms, which is a standard for coax cable used in the broadcast industry. You can use these cables with a 50 Ohm amateur radio antenna system, but because of the impedance mismatch, your SWR will be a little higher. For example, if everything else is balanced out, using the 75 ohm cable with a 50 Ohm antenna will increase your SWR to 1.5:1 at the transmitter.
For HF, this mismatch isn’t a big deal
Now this really isn’t a big deal as 1.5:1 is still within the 2:1 SWR limit that most modern ham radio transceivers will tolerate. In HF applications, the impedance mismatch can be further reduced by the use of an antenna tuner. The tuner gives the transmitter the 50 Ohm match that it expects. So for their HF antenna systems, many hams will use RG-59 or RG-6 because it’s cheaper than 50 Ohm RG-58 or RG-8, and any mismatch can be resolved by the tuner. It works, but it’s not an ideal situation.
But you also don’t have to live with the mismatch on VHF
VHF/UHF is another story. RG-6 is rated for applications below 150 MHz, so it can be used with transmitters and antennas on the 2 meter band. but above the frequency, the losses are going to be so high, that you might as well invest in quality 50 Ohm cable. While resolving the impedance mismatch isn’t critical (you can still run with an SWR of 1.5:1) most VHF rigs don’t offer antenna tuners to create a 50 ohm match for the transmitter. You’ll just have to take a small performance hit in exchange for saving money.
There are two ways to deal with this mismatch. They don’t require a tuner, but you will only be able to effectively use the cable run for a single frequency or band. By the way, these methods also work on HF too.
The first method is to measure and cut your coax so the entire cable run can be measured in 1/2 wavelength multiples. For the two meter band, a half wave is approximately 38 inches. Keeping your cable length within these 1/2 wave multiples will present a near 50 Ohm match at the transmitter end of the line. But how does this work?
Say you where to take a length of 50 Ohm coax and put a 100 Ohm resister at the end. If you where to measure the impedance at the other end, what would it be? Not necessarily 50 Ohms. The reason is that coax offers a mix of resistive and reactive elements that change with the length of the coax. For example, using 1/4 wave length multiples of 75 Ohm coax will give you your 100 Ohm resister a 50 Ohm impedance at the transmitter end. Now if you were to substitute that resister with a 50 ohm antenna, using 1/2 wave multiples of 75 Ohm coax would give you a 50 Ohm impedance. Not too shabby.
This practice works well with VHF as the 1/2 waves are relatively short, so you don’t need to contend with a bunch of extra coax cable. But as you lower the frequency, those wavelengths increase, to the point were you’ve got up to 120 feet of extra cable on the 75 meter band.
Another method is to use a transformer. Like I mentioned earlier that transmission line offers a mix of resistive and reactive components based on length, you can change the impedance of a 75 Ohm coax cable line by using short 1/12 wavelength sections of 75 and 50 Ohm coax at the feed point and transmitter. This method again only works for a particular frequency, but you will have enough bandwidth to still cover the 2 meter VHF band.what does zachary mean How to wrap odd shaped gifts? what does pregnancy feel like what does gonorrhea discharge look like what does it mean to hex someone How long does nike take to ship what are goblins what does tentative mean what does veracity mean How to convert mov to mp4 on mac what does power play mean in powerball How much is th e tips for hair dresser What is tips liver Are advertisers allowed to use non edible tricks when advertising food How to become a chef what does time is of the essence mean How to update hbo max on samsung tv? what does bona fide mean Tips for when you can't sleep How to makemask? How to pronounce colonel? How to record a phone call on android Tips on how to tackle writing a master's thesis what does torah mean How to make shrimp tacos what are wedding vows what does radius mean How to write a resume with no experience? How to can applesauce? How to chalk paint furniture Which one is taxed higher in texas? tips or salary increase Why do women keep faling for guy tricks reddit How to win rajshree lottery tips what does my phone number spell what are motor skills what does v6 mean Who tips the preacher at a wedding How to sleep through the night? what does the name luna mean what does on the lam mean what does karmic mean How to get windows 10 to show pop up tips How to find someone's birthday How much is a horse to buy How much on average does a pizza delivery driver make in tips How to clean an infected wound? what are kegels Best tips when buying land What is tricks power in lost girl How to cure gerd what does confer mean How to drain a ganglion cyst yourself How to cure sinus permanently what does dilation mean in math Where to buy inexpensive cake decorating tips How to sober up fast How to trick life 360 what are the 7 boroughs of new york city what does discombobulated mean What is the total percentage of people teaching there dogs tricks on there own in the usa what does jaundice mean what does 🤭 mean What kind of tricks can u do for feeding baby How much can you contribute to 401k? what does scribe mean How to remove quickbooks desktop export tips what does equine mean Tips on how to sell your house How to stop restless legs immediately at home what are the 48 countries in asia what time does dallas cowboys play How long does it take to become a lpn? How to draw mountains How to to promote a gay dating site tips tutorials what does sanctification mean How to make oatmeal cookies How to play 21 questions? How old is too old to have a baby Someone who playsa lot a tricks what does pro mean How fast do the tips of wind turbines spin what are playoffs what does a cherubim look like How to attach fur tips on christmas tree How to get gunpowder in minecraft what does emo mean How to do tricks on a fingerboard Why are the tips of my air plant turning brown what does high heart rate mean what time does home depot close today How to make cupcake boyquet with russian tips Do you have to answer if your employer asks how many tips you made? what does smoke do to bees Tricks on how to properly install a 3d bltouch what does cross street mean How to pick a lock what does the holy spirit do How to do magic tricks witb coins what does cs mean How to convert a fraction to a decimal Civ 5 how to turn off tips How to treat ear mites in dogs? what does engaging mean How to make a minecraft skin what does crossfaded mean How to start a farm what does alt mean in roblox How to read oil level on dipstick? How do i do the new snap chat tricks what does chingona mean what does yeet mean what does ship mean Old dog new tricks why How to find vertex Awesome tricky what are those tricks what does it mean to play by ear How much does it cost to create an nft what does user not found mean on instagram