|Definition||Talino, IZ7ATH has some very basic explanations of balun theory and construction|
|Theory, Design, Performance, Construction and Opinions on what works and why||
W0IYH Feed line Choke Performance (Tim, K3LR; Aug 18, 2003)
Coax balun on a PVC form (Ed, WA2SRQ;Aug 12, 1996)
K3LR and W0IYH "choke" baluns in the feedline system (Tom, W8JI; June 11, 1999)
An Inexpensive, High-Performance, Ugly 50ohm-Balun (Rich, AG6K)
C-31XR Balun (John, W7HQJ - Jan 13, 2001)
160m Baluns (Joe, W1JR - December 22, 2003)
|Vendors||Cal-AV Labs Inc||W2FMI high-power baluns|
|Product Reviews||From eHam.net -- here is what people have to say about various balun products|
May 25, 2005
by Paul B. Peters, firstname.lastname@example.org.
Local copies of reflector-based information presented below are simply a means to preserve the data for others to enjoy. The creator of this page recognizes the following information sources: TowerTalk reflector, Force-12 reflector, Rich Measures.
|Subject:||[TowerTalk] W0IYH Feed line Choke Performance|
|From:||email@example.com (Tim Duffy K3LR)|
|Date:||Mon Aug 18 16:19:16 2003|
I posted some of my experience concerning the W2DU type choke performance a few weeks ago. There were several requests for the test data. I retrieved my lab notes taken from my HP Network Analyzer on October 15, 2001. The W0IYH choke is made from 100 type FB-5622-43 beads on RG-142 with silver plated PL-259's on each end. The list is test frequency followed by impedance 1.8 MHz 1152 ohms 3.7 MHz 3483 ohms 7.1 MHz 4115 ohms 14.2 MHz 1783 ohms 21.2 MHz 1280 ohms 28.5 MHz 1234 ohms My tests with the W2DU choke: 1.8 MHz 984 ohms 3.7 MHz 1733 ohms 7.1 MHz 1921 ohms 14.2 MHz 1432 ohms 21.2 MHz 905 ohms 28.5 MHZ 423 ohms In 100% key down CW tests into a 50 ohm dummy load for 10 minutes I found the W2DU to overheat (individual bead temperature exceeded manufactures ratings) at 500 watts on every band. The W0IYH choke passed the same test at 2000 watts and was well within the temperature specification for each bead. I believe the W0IYH choke has adequate safety factor for 1500 watt stations as long as the VSWR does not exceed 3:1. There are lots of W2DU chokes in service and as you can see they will work well. The W0IYH design is an improved version. As I indicated in my September 1998 CQ Contest magazine article, I use the W0IYH design at my station. They are on every feed point of every antenna, at the tower mounted stacked antenna RF switch box and at the end of each antenna feed line where it connects to the RF amplifier in the radio room. They keep RF from flowing on the outside shields of the feed lines very well. If you are interested in ready to go chokes, completed W0IYH chokes are available from Comtek Systems. Please contact them for price and availability. http://www.comteksystems.com 73, Tim K3LR http://www.k3lr.com
Mon, 12 Aug 1996 08:40:24 -0400
Hi Pete, My experience is that PVC works fine as a form for high Q RF coils. I've measured Qs of up to 450 on loading coils wound on PVC pipe. I've appended a paper I wrote on measurements of coaxial baluns wound on PVC forms. 73, Ed Gilbert, WA2SRQ firstname.lastname@example.org --------------------------------------------------------------------- Having access to a Hewlett-Packard 4193A vector impedance meter at work, I have made measurements on a number of baluns, coaxial and otherwise. For my beams I was particularly interested how many turns and on what diameter are optimum for air core coaxial baluns, and what the effect of bunching the turns was (formless). Using the remote programming capability of the HP4193A along with an instrument controller, I measured the magnitude and phase of each balun's winding impedance at 1 MHz intervals from 1 to 35 MHz. For comparison, I also made measurements on a commercial balun which consists of a number of ferrite beads slipped over a short length of coax. I've appended some of these measurements so you can draw your own conclusions. PVC pipe was used for coil forms. The 4-1/4 inch diameter baluns were wound on thin-walled PVC labeled "4 inch sewer pipe". This material makes an excellent balun form. It's very light weight and easy to work with, and I obtained a 10 foot length at the local Home Depot for about 3 dollars. The 6-5/8 inch diameter forms are 6 inch schedule 40 PVC pipe which is much thicker, heavier, and more expensive. Each test choke was close-wound on a form as a single-layer solenoid using RG-213 and taped to hold the turns in place. The lengths of cable were cut so there was about 2 inches excess at each end. This allowed just enough wire at the ends for connections to the HP4193A's probe tip. After data was collected for each single-layer configuration, the PVC form was removed, the turns were bunched together and taped formless, and another set of measurements was taken. I have only included the "bunched" measurements in the table for one of the baluns, but the trend was the same in each case. When compared to the single-layer version of the same diameter and number of turns, the bunched baluns show a large downward shift in parallel self-resonance frequency and poor choking reactance at the higher frequencies. Interpreting the Measurements ----------------------------- All the baluns start out looking inductive at low frequencies, as indicated by the positive phase angles. As the frequency is increased, a point is reached where the capacitance between the windings forms a parallel resonance with the coil's inductance. Above this frequency, the winding reactance is reduced by this capacitance. The interwinding capacitance increases with the number of turns and the diameter of the turns, so "more is not always better". The effects of a large increase in interwinding capacitance is evident in the measurements on the balun with the bunched turns. This is probably a result of the first and last turns of the coil being much closer together than the single-layer coil. An important requirement of these baluns is that the magnitude of the winding reactance be much greater than the load impedance. In the case of a 50 ohm balanced antenna, the balun's winding impedance is effectively shunted across one half the 50 ohm load impedance, or 25 ohms. A reasonable critera for the balun's winding impedance for negligible common mode current in the shield is that it be at least 20 times this, or 500 ohms. The measurements show, for example, that 6 turns 4-1/4 inches in diameter meet this criteria from 14 to 35 MHz. The measurement data also reveals the power loss these baluns will exhibit. Each of the measurement points can be transformed from the polar format of the table to a parallel equivalent real and reactive shunt impedance. The power dissipated in the balun is then the square of the voltage across it divided by the real parallel equivalent shunt impedance. While this calculation can be made for each measurement point, an approximate number can be taken directly from the tables at the parallel resonance points. At 0 degrees phase angle the magnitude numbers are pure resistive. I didn't record the exact resonance points, but it can be seen from the tables that the four single-layer baluns are all above 15K ohms, while the ferrite bead balun read about 1.4K. These baluns see half the load voltage, so at 1500 watts to a 50 ohm load, the power dissipated in the coaxial baluns will be less than 1.3 watts, and the ferrite bead balun will dissipate about 13.4 watts (neglecting possible core saturation and other non-linear effects). These losses are certainly negligible. At 200 ohms load impedance, the losses are under 5 watts for the coaxial baluns and 53.6 watts for the ferrite beads. Conclusions ----------- - A 1:1 coaxial balun with excellent choking reactance for 10 through 20 meters can be made by winding 6 turns of RG-213 on inexpensive 4 inch PVC sewer pipe. - For 40 or 30 meters, use 12 turns of RG-213 on 4 inch PVC sewer pipe. - Don't bunch the turns together. Wind them as a single layer on a form. Bunching the turns kills the choking effect at higher frequencies. - Don't use too many turns. For example, the HyGain manuals for my 10 and 15 meter yagis both recommend 12 turns 6 inches in diameter. At the very least this is about 3 times as much coax as is needed, and these dimensions actually give less than the desired choking impedance on 10 and 15 meters. Measurements ------------ Magnitude in ohms, phase angle in degrees, as a function of frequency in Hz, for various baluns. 6 Turns 12 Turns 4 Turns 8 Turns 8 Turns Ferrite 4-1/4 in 4-1/4 in 6-5/8 in 6-5/8 in 6-5/8 in beads sngl layer sngl layer sngl layer sngl layer bunched (Aztec) ---------- ---------- ---------- ---------- ---------- ---------- Frequency Mag Phase Mag Phase Mag Phase Mag Phase Mag Phase Mag Phase 1.00E+06 26 88.1 65 89.2 26 88.3 74 89.2 94 89.3 416 78.1 2.00E+06 51 88.7 131 89.3 52 88.8 150 89.3 202 89.2 795 56.1 3.00E+06 77 88.9 200 89.4 79 89.1 232 89.3 355 88.9 1046 39.8 4.00E+06 103 89.1 273 89.5 106 89.3 324 89.4 620 88.3 1217 26.6 5.00E+06 131 89.1 356 89.4 136 89.2 436 89.3 1300 86.2 1334 14.7 6.00E+06 160 89.3 451 89.5 167 89.3 576 89.1 8530 59.9 1387 3.6 7.00E+06 190 89.4 561 89.5 201 89.4 759 89.1 2120 -81.9 1404 -5.9 8.00E+06 222 89.4 696 89.6 239 89.4 1033 88.8 1019 -85.7 1369 -15.4 9.00E+06 258 89.4 869 89.5 283 89.4 1514 87.3 681 -86.5 1295 -23.7 1.00E+07 298 89.3 1103 89.3 333 89.2 2300 83.1 518 -86.9 1210 -29.8 1.10E+07 340 89.3 1440 89.1 393 89.2 4700 73.1 418 -87.1 1123 -35.2 1.20E+07 390 89.3 1983 88.7 467 88.9 15840 -5.2 350 -87.2 1043 -39.9 1.30E+07 447 89.2 3010 87.7 556 88.3 4470 -62.6 300 -86.9 954 -42.7 1.40E+07 514 89.3 5850 85.6 675 88.3 2830 -71.6 262 -86.9 901 -45.2 1.50E+07 594 88.9 42000 44.0 834 87.5 1910 -79.9 231 -87.0 847 -48.1 1.60E+07 694 88.8 7210 -81.5 1098 86.9 1375 -84.1 203 -87.2 778 -51.8 1.70E+07 830 88.1 3250 -82.0 1651 81.8 991 -82.4 180 -86.9 684 -54.4 1.80E+07 955 86.0 2720 -76.1 1796 70.3 986 -67.2 164 -84.9 623 -45.9 1.90E+07 1203 85.4 1860 -80.1 3260 44.6 742 -71.0 145 -85.1 568 -51.2 2.00E+07 1419 85.2 1738 -83.8 3710 59.0 1123 -67.7 138 -84.5 654 -34.0 2.10E+07 1955 85.7 1368 -87.2 12940 -31.3 859 -84.3 122 -86.1 696 -49.9 2.20E+07 3010 83.9 1133 -87.8 3620 -77.5 708 -86.1 107 -85.9 631 -54.8 2.30E+07 6380 76.8 955 -88.0 2050 -83.0 613 -86.9 94 -85.5 584 -57.4 2.40E+07 15980 -29.6 807 -86.3 1440 -84.6 535 -86.3 82 -85.0 536 -58.8 2.50E+07 5230 -56.7 754 -82.2 1099 -84.1 466 -84.1 70 -84.3 485 -59.2 2.60E+07 3210 -78.9 682 -86.4 967 -83.4 467 -81.6 60 -82.7 481 -56.2 2.70E+07 2000 -84.4 578 -87.3 809 -86.5 419 -85.5 49 -81.7 463 -60.5 2.80E+07 1426 -85.6 483 -86.5 685 -87.1 364 -86.2 38 -79.6 425 -62.5 2.90E+07 1074 -85.1 383 -84.1 590 -87.3 308 -85.6 28 -75.2 387 -63.8 3.00E+07 840 -83.2 287 -75.0 508 -87.0 244 -82.1 18 -66.3 346 -64.4 3.10E+07 661 -81.7 188 -52.3 442 -85.7 174 -69.9 9 -34.3 305 -64.3 3.20E+07 484 -78.2 258 20.4 385 -83.6 155 -18.0 11 37.2 263 -63.2 3.30E+07 335 -41.4 1162 -13.5 326 -78.2 569 -0.3 21 63.6 212 -58.0 3.40E+07 607 -32.2 839 -45.9 316 -63.4 716 -57.6 32 71.4 183 -40.5 3.50E+07 705 -58.2 564 -56.3 379 -69.5 513 -72.5 46 76.0 235 -29.6
|Subject:||[TowerTalk] K3LR and W0IYH "choke" baluns in the feedline system|
|From:||W8JI@contesting.com (Tom Rauch)|
|Date:||Fri, 11 Jun 1999 09:18:38 -0400|
I've had some telephone line caused server problems, and I'm not sure my post made it to the reflector or anywhere. From: "Greg Gobleman" <email@example.com> To: "Tod-ID" <firstname.lastname@example.org>, <email@example.com>, "Bill Coleman AA4LR" <firstname.lastname@example.org> Subject: Re: [TowerTalk] K3LR and W0IYH "choke" baluns in the feedline system Date sent: Thu, 10 Jun 1999 20:02:45 -0500 > I also read W2FMI's book and I would have to agree that something isn't > right about the W2DU type Balun. I experienced heating and a rise in SWR > when using a KW and an under 2:1 SWR but not flat. It would heat up and > the standing wave would rise over 2:1. This is not to say that all bead > Baluns are bad. I had heard good things about the Force 12 version. > Perhaps it uses a different ferrite material. Walt's balun is based on good engineering for choking, but if you look at it closely there is no headroom for power. I suspect Walt never caught that because he mostly runs low power. There are certainly many cases where his balun would work OK, but 73 material or ANY material with high loss tangent is the wrong material for QRO or for use where the core is involved in handling any high flux density. > I built several of the W1JR type of Baluns and have had no problem with > heating. I have had a problem finding an inexpensive enclosure. I have > tried using 3" PVC caps and plugs and have about $5 in the enclosure. > However, I created another problem. Weight of the enclosure and the > core/coax with connectors is a bit much for a dipole. An inverted V or > mounting on a beam is not a problem. There is no need for the criss-crossed winding style, a single layer solenoid winding measures nearly the same. Some articles and books tell you any stray C across the balun reduces choking, but the opposite actually happens. You just have to be careful and not use such a large winding that the self-resonant frequency of the balun is lower than 1/2 of the highest operating frequency. The cheapest balun for a given impedance and power rating is still an air-wound coil of coax on a PVC drainpipe. > I have also had excellent success with a coil of coax. When ur lighting > every florescent tube within a block at 2 AM while on 80m with a flat SWR. > This will cure it. If you use multiple turns through a core, the impedance goes up by the square of the turns increase. If you stick them through a string of beads, the increase in impedance is linear with length and has almost nothing to do with bead thickness. An air wound choke is somewhere between unity ratio and squared impedance as turns are increased, depending on mutual coupling between turns. A string of 43 material beads 36 inches long has the same common mode impedance as a stack of 43 cores 1 inch tall with 6 turns of coax. The string of beads will handle more power, because it has more surface area exposed directly to cooling air no matter how thick the beads are (beyond a certain limit). The more stress the balun has, the lower the ui of the core you should use. At the feedpoint with high power, a low-ui low-loss- tangent core is generally best, like air or a 61 material. This is especially true if the feedline parallels the antenna, or if the element is off balance, or if the element impedance is high. In a coaxial line connected the normal way near the shack (like in the second chokes K3LR uses), a string of 73 material beads would almost certainly be acceptable no matter what the power level. The feedline should be grounded to the tower or another ground as soon as possible after the balun, only on the side of the balun closest to the shack if possible. I use air chokes, or 61 material cores at transmitting antennas. I use 73 or 75 material cores for receiving and in-the-shack or "down the cable a distance" isolation. 73, Tom W8JI email@example.com
An Inexpensive, High-Performance, Ugly 50ohm-Balun
Building a no-grief 1.8MHz to 30MHz 50ohm-balun is easy. No costly ferrite-cores are needed, just a short length of 3 to 5 inch size plastic pipe, about 25 feet of 50ohm coax plus some nylon cable ties. Solid-dielectric coax is best for this application because foam-dielectric has a tendency to allow a change in the conductor to conductor spacing over a period of time if it is bent into a tight circle. This can eventually result in voltage breakdown of the internal insulation. The required length of the plastic pipe depends on the diameter and length of the coax used and the diameter of the pipe. For RG-213/U coax, about one foot of 5 inch size pipe is needed for a 1.8MHz to 30MHz balun. For 3.5MHz to 30MHz coverage, about 18 to 20 feet of coax is needed. This length of coax is also adequate for most applications on 1.8MHz. The number of turns is not critical because the inductance depends more on the length of the wire (coax) than on the number of turns, which will vary depending on the diameter of the plastic pipe that is used. The coax is single-layer close-wound on the plastic pipe. The first and last turns of the coax are secured to the plastic pipe with nylon cable ties passed through small holes drilled in the plastic pipe. The coil winding must not be placed against a conductor. The name of this simple but effective device is a choke-balun.
Some people build choke-baluns, without a plastic coil-form, by scramble-winding the coax into a coil and taping it together. The problem with scramble-winding is that the first and last turns of the coax may touch each other. This creates two complications. The distributed-capacitance of the balun is increased and the RF-lossy vinyl jacket of the coax is subjected to a high RF-voltage. The single-layer winding on the plastic coil-form construction method solves these problems since it divides the RF-voltage and capacitance evenly across each turn of the balun.
A more compact, less ugly, 1 to 1 impedance-ratio, 50ohm trifilar-wound (with wire) ferrite-core balun could also be used but there would be some tradeoffs. Ferrite cores are not cheap. Also, the air-core of the coax-balun can't saturate like the ferrite-core and, unlike ferrite-core wire-wound baluns, single-layer wound coax-baluns almost never have an insulation breakdown problem. Also, a trifilar-wound balun does not like to work into anything but a perfectly balanced load. With an imperfectly balanced load, the coax-balun will not, as does the trifilar balun, generate a differential, third RF-current on the outside of the coax that brings the RF to the input of the tuner. The choke-balun is not fussy. It will work as well into a less than perfectly balanced load as it will into a perfectly balanced load, and do so without the possibility of creating a differential RF-current on the station ground and fricasseeing the operator's fingers.
|Subject:||C-31XR Balun ...|
|Author:||John Petrich <firstname.lastname@example.org>|
Thanks for the reply and comments about baluns. It is good to share ideas with people who make observations and have ideas.
Yes, I am familiar with the W1JR balun and have used it in some applications. It was good 30 years ago, it is still good today. The only reason that it is not as popular as it once was, is that the bead baluns are easier to construct and harder to goof up on. There may be a small advantage in terms of bandwidth for the bead baluns. In some applications, bandwidth is very important. In other applications, bandwidth is really not important at all.
I know what you mean about unrecognized balun heating. So many baluns are located up at the antenna feedpoint and the heating is only discovered after the balun has failed. Antennas can be properly constructed yet it is of major importance to pair the balun, the antenna and the band of operation correctly to avoid balun heating and unwanted feedline radiation. Feedline radiation isn't always a problem. Wanted feedline radiation can make for a useful antenna i.e. G5RV.
Balun heating is the result of common mode currents flowing on the outside of the coax shield. These currents are then dissipated in the real component of the complex common mode impedance characteristic for that balun. There is no other source for heating for the ferrite beads. This heating problem occurs just the same way and for the same reason with all ferrite baluns, whether they are constructed with ferrite toroids or ferrite beads. The phenomenon is the same. It is interesting, if you carefully examine an overheating bead balun, the beads closest to the high impedance connections are the warmest. The beads closest to the low impedance connections are the coolest. It is as if each little bead functions as an individual little attenuator element. The entire stack of ferrites does not act like a resistor. The power from the common mode current is not dissipated uniformly as it would along a purely resistive element.
There are two independent factors that contribute to common mode current flow and the resultant risk of balun heating:
1) INSUFFICIENT COMMON MODE IMPEDANCE TO CHOKE OFF COMMON MODE CURRENT FLOW: Anytime, repeat "anytime", one of these 800 ohm common mode impedance bead baluns is connected across a high impedance load, such as a 80 meter doublet excited on 40 meters, there is the risk of severe balun overheating. The same goes for trying to operate a old style tribander on 17 or 24 meters with a ferrite balun. Low power operation won't heat the balun, BUT, the common mode current is still flowing, and the system could be operating at a disadvantage. This limitation from the balun's common mode impedance in a high impedance environment is BY FAR THE MOST SIGNIFICANT FACTOR that contributes to bead balun overheating. High power makes the heating problem easier to recognize. Low power doesn't cause as much heating but the system may not be functioning in an ideal manner. But, "everything works." A better solution for a balun in a high impedance enviroment is to use one of those coiled coax or "Badger", baluns. This particular style of balun is capable of exhibiting extremely high common mode impedance values if properly constructed and tested for the frequency of use. Just like an old antenna tuner of years gone by.
2) FERRITE MIX: Yes, ferrite mix can make a difference, but don't get overly excited on this one. Any importance that ferrite mix has on balun heating is not because one mix is "better" than another, or one mix is "worse" than another. The reason that ferrite mix can contribute to balun overheating problems is because of #1 above- Insufficient Common Mode Impedance. The Force-12 balun, I'm guessing, acts like a string of #43 mix ferrite beads. The Maxwell, W2DU, bead balun uses a string of #77 mix ferrite beads. The Force -12 balun has a good peak common mode impedance from 40 meters to 10 meters. The Maxwell bead balun has a useful peak common mode impedance from 160 through 15 meters. There is substantial overlap for both and both are good. The Maxwell balun might not have enough common mode impedance on 10 meters and overheat in some 10 meter applications. The Force 12 balun might not have enough common mode impedance for a 160 meter installation and overheat in some applications on that band. I haven't actually tested each balun side by side in the antenna situations I have referred to but I am extrapolating from their common mode impedance curves.
The key to reducing balun overheating probably lies with pairing up the antenna (and it's feed point impedance), and band of operation, with a balun having sufficient common mode impedance to choke off common mode current flow. The standard of comparison between "current mode" baluns is their measured common mode impedance at the frequency of use. Some "current mode" baluns have low common mode impedance compared to other baluns. I have only tested the Force-12 and Maxwell baluns and they exhibit common mode impedances of about 800 ohms. Unfortunately, the various manufacturers never publish the common mode impedance characteristics of their baluns. I think that it is very very very hard to get common mode impedance values greater than 800 to 1000 ohms using low Q type #43 and #77 ferrites. Maybe I don't know enough, so take that statement with a grain of salt. One can get relatively high common mode impedance by coiling coax on a higher Q #61 ferrite toroid. The air coiled coax, "Badger, balun or an old fashioned antenna tuner will give the highest common mode impedance values that I know of.
Let me know your thoughts, Greg.
John Petrich, W7HQJ
|Subject:||Re: Topband: 160 Meter BALUNS|
|From:||Joe Reisert <W1JR@arrl.net>|
|Date:||Mon, 22 Dec 2003 10:24:03 -0500|
A follow up to my prior EMail on a solenoid baluns.
Regardless of whether 50 feet of RG8X wound on a solenoid (tube) is proper to use a balun or not, I'll leave that up to you and W8JI to decide. I guess you some would describe this type of balun as a choke.
Now, here is some more theoretical information and measurements etc. that maybe of interest to some for the engineering types on this reflector. This maybe helpful to design similar types of solenoid baluns at other frequency bands.
50 feet of RG8X coax is a good starting point for a 160 meter solenoid (choke) balun for many reasons. If you close wind the coax on a 4.5" OD (I mistakenly said 4" in my prior EMail) standard white PVC tube, you obtain an impedance of about 650 Ohms (as measured on an HP Network analyzer). This means that you are above the 500 Ohms (and well above 250 Ohms) impedance that most experts feel is adequate for a balun impedance.
At 80 meters, the same solenoid balun will have an measured impedance of about 1300 Ohms. However, depending on how tight you make the turns, a resonance will be noted somewhere between 12 and 15 MHz. Hence, 50 feet of coax is probably only good for 160 through 30 meters. Use less turns if only for higher bands (see below). It looks like a 4 or 5:1 ratio of lowest frequency to highest useable frequency is a good rule of thumb.
To carry on further, 50 feet of RG8X will amount to approximately 38 turns on a 4.5" OD former and the winding will be approximately 9.25" long. If you plug these numbers is into most standard equations to calculate inductance, you will calculate an inductance of approximately 60 micro Henries. Using the standard formula for reactance: Xl = 2 pi FL, yields about the same impedance as measured above. Pretty nifty to get such agreement! So, you can see that don't need fancy measuring gear to make a solenoid balun for any band. Just decide on how high an impedance you want (but not too high-see below) and make sure that you don't put on too many turns so resonances will occur above rather than in band!
Some may ask if RG58 is OK for a solenoid balun. Sure it is but for lower power than RG8X. Since it is slightly smaller in diameter, 50 feet have a slightly higher impedance. Since the power handling ability of coax goes down as frequency increases, it maybe safer to use Teflon (RTM) type coax such as RG303 if you are running high power, especially at 80 meters and above. RG8 will also be OK but it is larger in diameter so more coax will be required. You can make your own calculations on this one. I wouldn't recommend foam RG8 coax as it may deform on such a small diameter. However, if you use a larger diameter tube, that will work with RG8 and since the diameter is larger, the impedance will increase accordingly. Use the standard inductance equations.
I made a solenoid balun with 25 feet of RG303 teflon (RTM), about 20 turns on a 4.5" tube, and the first measured resonance was about 24 MHz. This balun would be great, even at high power, for 80 through 15 meters. Again, about a 5:1 frequency range.
Some purists will say to space the turns, for example, by the diameter of the coax. This maybe less of a problem for flash over if lightning hits. I'll leave that up to you to decide. However, using the info above, this would calculate (using standard inductor equations) to about 33 micro Henries of inductance for space windings (and an impedance of only 375 Ohms), well below that normally suggested for a 160 meter balun. Hence, more coax or a larger diameter tube is required.
Finally, what about laying the balun on the ground. I'd recommend against that simply because that at least may lower the self resonance frequency. This is the old story that you shouldn't place objects near (1-3 diameters away) an inductor (which is what a solenoid balun is on the outside shield).
I hope this info is of interest and help. There will always be the disagreements over whether to use ferrite beads, ferrite toroids or solenoid baluns. No one size fits all! However, for those interested in designing their your own solenoid type baluns, I've hopefully given some info on how to "roll your own."
Happy holidays and best of DX in 2004.