HF Mobile gettin serious

Transcription

1 HF Mobile gettin serious I had some big noise problems on my HF rig using my new monoband centre loaded mobile whips. I read K0BG s excellent website dedicated to mobile issues and decided it was finally time to get serious and sort it all out. Previous to this my only experience of mobile radio was for a couple of years of CB in 1981/82 and about 5 years using VHF/UHF in the late 1990 s. Setting up a mobile for VHF i.e. 2m and 70 cm is relatively easy but HF mobile, particularly on 20m and below is a whole new ball game!! I did some serious reading on how to set up HF mobile and followed lots of advice & info from K0BG s site (k0bg.com). Strangely enough, bonding the various car body panels together and to chasis earth significantly reduced engine and other noises. I can comfortably listen to HF QSOs with ease whilst driving. Unless I am picking up airborne interference, I am able to copy signals previously lost in the noise. For my particular mobile installation there was a massive, massive reduction in noise. The second part of this exercise was to tune my mono band centre loaded whips for resonance and low VSWR. Once again, I felt at times I was rushing head over heels down the rabbit hole it was all new.

2 I also wanted a way of mounting the antenna at a retracted angle so I can get under my garage door and under those supermarket maximum height barriers and listen to my band of interest when mobile. When the antenna is retracted (see image above) signals are way down between 3 6 S points but, I am still able to copy the band whilst on the move. When I hilltop it takes a few seconds to put the antenna into its upright position. (1) I found that setting up HF mobile is very different to setting up an HF base antenna. The most efficient place to plonk your antenna is through and at roof centre (if possible). Generally speaking, the more metal under the base of the antenna, the more efficient it will work. I placed my antenna mount slightly forward of roof centre to the front of the car which means that I am aware that maximum radiation takes place towards the back of the car, along the plane of maximum metal length. The car body is best thought of as a capacitor between the antenna and ground due to the car wheels sitting between the two. This concept never occurred to me. (2) I ve always avoided drilling through the roof but this time I decided to do it and guess what, it s much neater with no cables sticking through the door rubbers. The coax is hidden behind the door pillar upholstery. Before you put the headliner back up, hook up your coax and ensure continuity between the centre of your coax and the radiating bits of your whip. Also ensure continuity between the base of the whip mount and the ground of your vehicle. You can see the old marks from my trusty mag mount on roof centre but these marks will go when I next clean the car!! (3) Tuning and matching an HF mobile whip takes two stages. Adjust whip length for resonance at your chosen frequency (its tune) and determine lowest VSWR (its match). Without using a shunt or other matching coil be prepared to faff about when determining VSWR. Antenna resonance point and lowest VSWR point inevitably fall on different parts of the band frequency for a number of reasons but this is not the place to delve deeper into why (see k0bg.com for reasons why). Depending on ground and other losses, the input impedance of your antenna setup at best is approximately 5 35 ohms depending on frequency band. You can raise this up to approximately 50 ohms by use of a shunt coil or other matching means. I also found that because of elusive ground losses and other unquantifiable variables, it s really difficult to understand what s going on in terms of resistive and reactive loadings. In other words, the input impedance presented to the rig is a motley combination of losses,

3 mostly ground losses, capacitive, reactive losses and coil losses. Within all these losses is the most important loss Rr (radiation resistance) and this is the one loss you want to increase because it refers to your radiated signal!! Mobile HF can be problematic and its not too surprising if you think of the physical length of a mobile whip which is much below a ¼ wavelength. Efficiencies for an 80m mobile whip approximate 1% so you can easily understand that mobile whips are a compromise but, HF mobile is great fun and well worth the extra effort to sort its efficiency onwards and upwards. (4) During tuning I completely disregarded the R= and X= readings on my MFJ 259B antenna analyzer for 20m and 40m and relied purely on readings from the resonance mode of the analyser. (more on tuning later). Ten metres appeared to load up really well though and I set resonance for MHz with a plus or minus VSWR <2 of about 750 khz bandwidth. From my very limited experience of installing antennas on HF/VHF mobile I think the 10m band is borderline in terms of tuning/matching problems because the input impedances without any matching device was approximately ohms. This equates roughly to a VSWR between ish. I would also be wary about using just an VSWR meter because SWR meters detect relative voltage only and a combination of resistive and reactive loadings may appear as a low VSWR and you may think you ve cracked it but it may not be so. (a) Firstly though some basics such as battery and bonding. I connected the positive cable to the rig direct to the battery ensuring the cable was fused. The negative side of the rig cable was connected to the thick multiple earth point strap on the chasis. Just find the thick earth strap that comes from the negative side of the battery and connect your negative cable where it bolts onto the chasis. My car is a negative earth return circuit. (b) I bonded all doors, bonnet (both sides) and tailgate, both sides (see Figs 2 and 3 below). I also bonded both sides of the engine to the chasis. Bonding is necessary in order to ensure that you maximize capacitor/ground surface as much as possible. Bonding is important for two reasons, (1) reduces noise big time and (2) antenna tuning is slightly easier when you have a bigger surface area underneath the antenna. Before you bond everything together, check resonance of your antenna, make a note of its frequency and repeat resonance measurement after bonding i.e. you ll find that resonance will shift to a different frequency. The only items left for bonding on my install is the exhaust system and a few other points on the engine. I will do this asap.

4 (c) I also built a whine filter detailed here to smooth out any spikes in battery voltages (see images below). The whine filter effectively reduced that heavy thump noise which comes and goes as you accelerate/decelerate. Following all these changes I finally got around to tuning the antenna. As pointed out earlier, there is an awful lot going on when attempting to tune an HF mobile whip. Please check out K0BG s site on how best to proceed. TUNING: Firstly, I roughly tuned my 20m, 10m and 40m based on the manufacturer s recommended whip lengths. For this I assumed that each whip was resonated for mid-frequency of each band and all I need do is check VSWR. Best match on 10m was between 1:1.2 and 1:2.0 (750 khz bandwith) Best match on 20m was 1:2.5 and 1:4.5 (150 khz bandwidth) and best match on 40m 1:3.5 and 1:7.5 with just 90 khz bandwidth. From these readings I reasoned that ground losses increase as I decrease in frequency due to input impedance approaching freefall on 40m. I expected this and it meant that thus far, everything is working more or less as it should. I then tuned it properly for resonance and match. I rarely get excited and do not waste an awful lot of time faffing about obtaining a low VSWR because it s usually just a 50 ohm impedance load between antenna, coax and rig. But this time I had to take it seriously and adjust for lowest VSWR using capacitive shunt matching. By the way, resonance point and lowest VSWR point do not coincide, the point is to get them as close together as possible. For each whip (using my MFJ 259B antenna analyzer) I adjusted whip length for resonance at my chosen mid band frequency. I knew that (a) resonance is the point of maximum RX/TX and (b) there is no point in measuring VSWR at this stage of the game due to the inherent low impedance of a typical mobile whip setup. I need a shunt coil here between the inner conductor of the coax and a ground connection to raise the impedance up to 50 ohms. I could build one or buy a shunting coil but, I use 3 mono band whips and although 10m is good to go I needed a shunt coil that will effectively match on 20m and 40m. I guess this is do-able but the only place I can put the shunt coil is just above the C-pillar resting against the top of the seatbelt fastening point. In other words, the shunt coil will be located inside the vehicle just above head height!! I decided to buy an MFJ position shunt coil and it has 6 adjustable positions using a bypass position, 120pF, 270pF, 470pf, 1000pF and 1500pF value capacitors. The 20m whip has a VSWR match under 1.7 for about 250 khz of bandwidth using a shunt value of 120pF and the 40m whip has a VSWR match

5 of and about 125 khz of bandwidth using the same shunt value, (40m is a bit strange and I will need to look into this more). The MFJ matching device should be located no more than 24 inches from the base of the antenna and my install is at the extremes of this measurement sited atop the C pillar. I guess instead of using a shunt coil you could simply use a coupler to match your whips or if you decide to mount your antenna at the rear of your vehicle you can build a shunt coil and place it directly under the base of your antenna mount between the centre conductor and a good earth point. Conclusions: I have not had chance to TX yet but RX but noise levels are down and signal strengths for local EU stations are good even with the whip angled towards the roof. Asia, Japan and Indonesia (for some reason) are really strong although I need a much longer time to get used to the new antennas before I can comment properly on relative signal strengths. To summarise, I have significantly reduced engine and other noises. The audio quality is much better and I am able to copy much weaker signals than before. Signal strengths are higher as well and I am able to copy band conditions reasonably well even with the antenna retracted at quite a low angle. I now have a radio that sounds like a radio and not a bag of whistling nails. A few construction details and images can be seen below. Quick and cheap way to brew your ground straps Go to your local hardware store and buy those aluminium covered water pipes you normally find under your kitchen sink and cut both ends off.

6 The next stage is to squeeze the inner rubber hose in a vice and gently ease the outer strap off with your hands, (best use gloves for this bit). Once you have the strap bit on the desk, flatten it and poke a small hole through each end so that you can put bolts, washers etc. See K0BG s site on proper way to attach the straps to the car body with serrated star washers etc. You can also use double stranded RG213 outer braiding to make ground straps because copper has a lower impedance than aluminium i.e. presents a lower impedance to ground but I was also interested in mechanical strength and Al is the way to go.

7 To simplify matters (I do not like working on cars) I decided to use existing drilled holes because it was too much hassle drilling new holes and at heart I am very much an amateur, amateur (see image below). Details on the whine filter and how to make it and connect up to your rig can be found here: I looked in my junkbox but did not have a 35v 4700 uf capacitor so I took a 35v 2700 uf capacitor out of the PSU board of my Yaesu FT-736R. I then connected 2 other capacitors both 2200 uf in parallel to give me approximately 7000 uf capacitance. (BTW, I will be re-building my old 736R PSU board with all new components as a winter project)!!

8 I then put the whine filter in a plastic box and it was good to go. This is my first proper attempt at HF mobile installing and so far so good. I need to do much more work to get it working better and currently thinking about getting some whips with big fat Yankee style low loss coils. I also need to bond my exhaust system and a few more selected points within the engine bay compartment. I would also like to try a long whip with a capacitance hat for 40m and 80m (parked mobile only) but for now though, I just want to play at my favourite hilltop site.