Direct Coax Feed for Comtek Vertical Antennas
A common question that I receive is what type of coax cable should I use for my J-Pole
antenna.
This is a great question, as the coax you select can make a dramatic difference on the performance and range of your antenna system. There is a wide variety and choices of coaxial cable available and each type serves a different purpose. This article highlights the most common types of cable and which ones are ideal choices for VHF/UHF antenna operation.
All coaxial feedline cables will exhibit signal loss. The amount of loss is dependent on three factors, the construction of the cable, the frequency of the signal, and the feedline length. Balancing these variables is crucial in getting a solid signal out of your antenna system. For example, RG-58 is a very common cable and readily available at Radio Shack and CB shops. But RG-58 has a high loss factor in VHF and UHF frequencies, so it really only is useful in runs less than 50 feet for frequencies above 100 MHz, and 20 feet for frequencies above 400 MHz.
Lower loss cable like RG-8U is great for 50-75 foot runs in the VHF/UHF range. It is reasonably affordable and easy to work with. Most ham radio stores or online merchants like The Wireman or Cable Xperts carry RG-8U.
If your cable run is going to be longer than 100 feet, or you are installing a UHF antenna for 70cm amateur radio, public safety, or GMRS; then I would recommend a low loss cable like LMR-400, Belden 9913, or similar shielded low loss cables. These cables have a heavier center conductor and an additional layer of shielding to better carry high frequencies with minimal loss.
What about RG-59 or RG-6 cables? These types of coax cables are commonly available and primarily used for cable TV installations. But they are matched for 75 Ohm television systems and will have an extra loss factor when used on 50 Ohm radio communication systems. You can use RG-6 with your radio, but because of the mismatch, 50% of your signal will be instantly eaten up by the cable. That's like turning a 50 watt radio into a 25 watt radio without even factoring the feedline loss yet. You can do it, but I don't recommend it.
Feedline loss is measured in decibels (Db) per 100 feet. The decibel scale is logarithmic, so a loss of 3 Db is a 50% reduction in power. A 6db loss is 50% on top of 50%, and so on. For example, if you have 6 db of feedline loss, your 50 watt signal coming out of the radio will be 12.5 watts as it enters the feedpoint of the antenna. The good news is that if the antenna has 6 db of gain (like the KB9VBR Slim Jim Antenna) the signal will be multiplied so that as it exits the antenna, it will be near 50 watts again. To calculate your total losses or gains, you subtract the feedline loss and add the gains from the antenna. The net result is your total gain or loss.
The chart below illustrates the feedline loss for most common coax types used for VHF and UHF radio communication. Db loss values are per 100 feet, so you'll need to adjust the loss figures based on the actual length of your feedline.
| Feedline Type | Loss in Db/100 ft 150 MHz | Loss in DB/100 ft 450 MHz |
| RG-58 | 6.2 | 10.6 |
| RG-8X | 4.7 | 8.6 |
| RG-8U | 2.8 | 5.2 |
| Belden 9913 | 1.5 | 2.8 |
| LMR-400 | 1.5 | 2.7 |
Finally, in shopping around, you may notice that the feedline run is more expensive than the antenna. When installing an antenna system, you greatest expense is usually in the feedline. A wise ham told me several years ago to never skimp on feedline, but instead install the best that you can afford. The best antenna in the world will sound terrible with an inefficient feedline, but an average antenna will always outperform if it's fed with top quality coax.
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Source: https://www.jpole-antenna.com/2013/01/12/what-type-of-feedline-coax-should-i-use-for-my-antenna/