RG-6 TV Coaxial Cable for HF Transmission Lines

Transcription

1 INTRODUCTION RG-6 TV Coaxial Cable for HF Transmission Lines Looking for low cost co-ax for long HF feeder runs? The author needed to hook up to a 40 m inverted V located about 200 ft. from the shack, and the ready availability of RG-6 type 75 ohm coaxial seemed to offer a potentially attractive low cost solution. The typical outdoor rated or "indoor/outdoor" cable available at the usual hardware stores has a black PVC outer sheath and is sold either as bulk cable or pre-packaged rolls and is generally marked "2,400 MHz Digital Satellite". The inner conductor is 18 AWG copper plated steel and the outer conductor is a combination of a thin aluminium foil sheath closely bonded to the inner dielectric material with an outer reinforcing braid that seems to be made from thin plated steel wire. If it's OK at 2,400 MHz it should be OK at 7 MHz, at least for moderate power levels - right? The following describes the results obtained with various examples of RG-6 as well as some practical advice on using this type of cable in nominal 50 ohm systems. MEASUREMENTS The author purchased two 30 m reels of pre-terminated "2.4 GHz rated" RG-6 coaxial cable (OMEGA OVK7-30) from a local Home Hardware store to connect to the aforementioned 40 m inverted V. It was of course realised that the match would be less than perfect and some SWR could be expected on the transmission line. Measurements using a MFJ-259 with a short length (30 ft.) of known good RG-8 coax connected to the antenna showed the expected wide swings in SWR from about 1.7:1 at resonance to full scale (well above 3:1) away from resonance. When the SWR measurements were made at the shack with the additional 60 m of RG-6 in line the results were surprising. Instead of an expected dip somewhere near the originally measured resonant frequency the meter just indicated an SWR varying between about 1.5:1 and no more than 3:1 over the HF range with the SWR dips being fairly broad and bearing no relation whatever to the expected antenna fundamental resonant frequency or any of its harmonics. All this pointed to a lossy feedline so, after checking the DC continuity of the two series connected 30 m cables, attenuation measurements were made on one of the cables. The maximum frequency at which reasonably accurate measurements could be made was 50 MHz, with the results given in the following table, which includes for comparison the characteristics for "standard" RG-6. (The OVK cable length is actually 30 m, or 98 ft. so attenuation measurements for this cable are effectively the db/100 ft. values.) The attenuation values for the "standard" RG-6 were obtained from The Times Microwave Coax Calculator at This calculator is a useful resource for comparing coaxial cables, but bear in mind that the values obtained from different sources are not necessarily all the same so use caution when interpreting. If possible use the manufacturer's information for specific cables if Page 1 of 5

2 you can identify the details from the type number. There's really no such thing as a "standard" cable. Frequency MHz Attenuation db/100 ft. OMEGA OVK7-30 Standard RG At 7 MHz frequency the attenuation of the two Omega 30 m cables in series was measured at around 6 db for a matched 75 ohm load. (Extrapolating to attenuation at 2,400 MHz, the loss would be around 80 db / 100 ft. unless there is some other mitigating factor connected with the thickness of the copper plating on the steel centre conductor and reduced skin depth penetration as the frequency increases. However this is outside the scope of this investigation, which is concerned with operation of the transmission line at HF.) The Omega cables were then replaced by an equivalent length of bulk RG-6 Ethernet / CATV cable (Comspec ). This cable happened to become available from scrap supplies shortly after purchasing the Omega cable, and if used initially would have saved a lot of trouble as well as the need to pen this article. Repeating the attenuation measurements using the Comspec RG-6 resulted in a value of 1.5 db / 100 ft. at 50 MHz, which is only slightly worse than the nominal 1.4 db / 100 ft. rating of standard 50 ohm RG-8 at this frequency, and actually better than the "standard" RG-6 attenuation of 1.87 db. At 7 MHz the attenuation of the entire 200 ft. RG-6 cable run was too small to measure with any precision, but estimated to be at around 0.3 db. Physically there is very little external difference between the two types of cable. The braiding on the Comspec cable is slightly denser than that of the Omega cable and the aluminium foil is very tightly bonded to the inner dielectric. In the Omega cable the foil can readily be peeled away from the dielectric. The dielectrics and inner conductors themselves appear identical. It is likely that the differences in HF RF performance relate mainly to the quality of the dielectric insulating material so there's no reliable way of visually differentiating between good and poor cables of this type. The DC resistances were measured at around 2.5 ohms per 100 ft. for both the inner and outer conductors of either cable type, so simple resistance measurements don't help much either. Page 2 of 5

3 Another pre-terminated cable type tested, but not used due to having only one 100 ft. roll available, was the GE RG-6 purchased from Canadian Tire (product code 20638). This cable proved to perform midway between the Omega and Comspec cables, with a measured loss of 3.25 db / 100 ft at 50 MHz. The Omega and GE RG-6 cables are made in China, whereas the Comspec cable was almost certainly of North American manufacture, but appears to be no longer available. SWR EFFECTS Attenuation in any long coaxial cable run is minimised when the cable is terminated by a resistive load equal to its characteristic impedance, and therefore operates with a standing wave ratio (SWR) close to 1:1. Section 19 (Transmission Lines) of the ARRL Handbook provides an excellent account of the effect of added losses due to SWR effects. Correct matching is also important if you are using high power in order to minimise the peak voltage on the transmission line. The following describes how the 40 m inverted V was matched to the Comspec CATV RG-6 cable. A "T" type L-C tuner was inserted between the 30 ft. of RG-8 connected to the antenna and the remote end of the 200 ft. RG-6 line from the shack. The purpose of the tuner is to match the antenna / RG-8 coax combination to 75 ohms. Now the MFJ-259 is designed to measure 1:1 SWR in a 50 ohm system, so when it's used in a 75 ohm system the tuner is adjusted so that the minimum SWR is 1.5:1 (NOT 1:1) and the corresponding resistance reading is around 75 ohms. Thus the MFJ was connected to the tuner in place of the 75 ohm RG-6 cable and the tuner adjusted as above. To check the results the RG-6 was reconnected and the SWR checks repeated in the shack. The SWR and resistance readings remained essentially unchanged, indicating that the cable was looking into something close to the required 75 ohm terminating impedance. If you are using an analyzer that measures SWR only then the tuner still needs to be adjusted for a 1.5:1 SWR dip but there will probably be two settings of the tuner where this is achieved. To check which is the correct tuning point insert a length of 75 ohm coax between the tuner and the SWR meter (an electrical length of around 0.25 wavelength is most suitable, but not critical) and re-check the SWR reading. If you have the correct resonance the SWR should remain at 1.5:1. MATCHING TO 50 OHMS If you are concerned about the 1.5:1 SWR that the matched 75 ohm cable presents to the standard 50 ohm transceiver antenna port, then a tuner or a series section match can of course be used to present a 1:1 SWR load. However most solid state transceivers and virtually all tube type transmitters or transceivers will work quite happily into a 75 ohm resistive load impedance. A series section match for interfacing between 75 ohm and 50 ohm transmission lines or loads is shown in the sketch below: Page 3 of 5

4 75 ohm line (any length) or 75 ohm load 300 ohm twin lead (23 degrees) 2.29 m at 7 MHz (0.85 velocity factor) 75 ohm coax (79 degrees) 6.24 m at 7 MHz (0.67 velocity factor) 50 ohm line (any length) or 50 ohm load The example assumes 300 TV type twin lead and typical 75 ohm RG-6 coax. Dimensions can be scaled for other frequencies and other cable velocity factors. POWER HANDLING There appears to be no definitive information available on the power handling capability of the TV/ Ethernet type RG-6 cables. The principal dimensions of the Comspec cable are: Inner conductor: Dielectric: Braid: Overall: in. dia (18 AWG copper plated steel), in. (including thin bonded Al. foil) in. (over outside of braid) in. The other cables examined here were similar in respect of the above dimensions. For comparison, standard RG-6A/U cable is listed (ITT Reference Data for Radio Engineers, H. W. Sams, 5 th. edition 1969., Section 22) as having a 21 AWG copper coated steel centre conductor ( in. dia.) and in. dia. dielectric. The maximum RMS voltage rating for solid polyethelene (PE) dielectric RG-6 cable is 2700 volts compared with 3700 volts for PE insulated RG-8 and 600 volts for foam polyethelene (FPE) insulated RG-8. TV type RG-6 generally uses a FPE dielectric, so extrapolating the working voltage from the RG-8 specifications would suggest 440 V RMS as the maximum for this type of cable. The corresponding maximum power rating is given by: 2 E P = Z 0 SWR Where E is the RMS voltage, Z 0 is the characteristic impedance (75 ohms) and the SWR is effectively a de-rating factor. For unity SWR the maximum or peak power rating for TV type RG-6 is therefore likely to be of the order of 2.5 kw. This peak power rating corresponds to the maximum useable PEP power for a SSB transmitter or the key down power for CW and is independent of frequency. However this does not account for the power rating of the cable, which is limited by thermal considerations, and is highly frequency dependent. Page 4 of 5

5 The Times Microwave Coax Calculator produces the following results for the power handling capabilities of standard RG-6 and RG-8 cables; Frequency (MHz) RG-6 (kw) RG-8 (kw) The TV type RG-6 is likely to be similar in respect to power rating as compared with the standard RG-6 listed above, but that is about all that can be said lacking any more definitive information. Clearly RG-8 is a better choice for higher power operation. Note that the power levels above must be de-rated by the SWR if the SWR departs from 1:1. For example if the SWR is 2:1, then the maximum power levels are halved. The normal continuous carrier power that the author's 40 m AM transmitter applies to the 75 ohm transmission line run is around 160 Watts. The peak envelope power on modulation peaks is 640 Watts. There has never been any instance of breakdown in the 200 ft. RG-6 cable run under these conditions. The same RG-6 cable run has also been used as a temporary measure to feed a mismatched 16 ft. vertical antenna which presents a SWR of about 3:1 at 18 MHz when measured at the sending end of the cable. With the SWR matched by a tuner in the shack and a transmitter running SSB at 500 W PEP into the tuner there was no evidence of cable breakdown. In this case the total matched line loss was estimated to be 2.2 db at 18 MHz, with an additional 0.8 db loss due to the poor SWR. CONCLUSION The principal lesson from all this is that the hardware store variety Chinese manufactured pre-terminated RG-6 cables can not be trusted to provide attenuation even close to the nominal 2 db / 100 ft. 50 MHz rating of standard RG-6. They may have an application as lossy termination cables for rhombic antennas and terminated Vee beams, but should be avoided for general amateur use. Bulk exterior quality RG-6 cable as used by professional satellite or cable installers is likely to be of much better quality, as of course should be the case for RG-6 cable purchased from any reputable amateur dealership. Provided the RG-6 is of acceptable quality from the attenuation point of view, then the power handling in a reasonably well matched transmission line run should not be a concern at moderate power levels up to 30 MHz. Matching to a 50 ohm system is fairly straightforward and, even without precise matching, the residual SWR is often of little consequence. Page 5 of 5