Ant Qs...

[Todd, KA1KAQ]

Okay....bear with me as it's been years, and I was never too terribly bright on this stuff to start with.

Got the new 80m dipole up on Sunday. It's a beast, has one of those B&W CC-50 center insulators, heavy gauge stranded, insulated wire (#10 or larger), and 125 feet of RG-9B (aka RG-214) 50 ohm double-shielded coax. Cut to 3700 or so. Should last forever.

After testing on several different frequencies, I noticed the antenna current was an amp or more higher than my previous marginal antenna cut for somewhere around 3885. 'Marginal' in that the wire used was bare copper stranded, smaller gauge (14-16, at largest), old foam coax stub (maybe 15 feet) soldered directly to an old porcelain center insulator, then spliced to a run of 9913.

Antenna current is great right around the frequency it's cut for, obviously. Ant current is under 4A @ 3710. But when I tune it up to 3870-3885, it goes up around 1-1.5 amps to 5-5.5 amps.

Here's the kicker: the crappier old ant was much shorter, had been spliced, rubbing on branches, old foam coax spliced to newer Belden, etc but didn't exhibit much change when I moved it down the band to 3725/3710. It was about 12 years old when it went back up in April of '06. Seemed to stay at 4A ant current across the band.

So I'm confused. I now have a nice, solid, heavy duty antenna made of continuous lengths of wire, insulated from shorting on branches (it's in the clear anyhow), a solid run of double-shielded feedline maybe 25 feet longer, 2 ohms less impedance (50 vs 52) with the excess wound into about a 1 foot spool and taped (ala choke-balun).

It strikes me that this ant should be more broad-banded, not less. Yet I seem to be seeing the opposite. The old ant had no ferrite beads, no choke-balun, no tuner beyond the transmitter's Pi-L output, nuttin'.

I can certainly trim it a bit for a better average between high and low limits, but I'd really like to understand what's going on. Considering that the ant is pretty straightforward, the feedline strikes me as the culprit. Could going from 52 to 50 ohms and adding some extra length really change things that much? Feedpoint of an ideal half wave dipole up the ideal height above ideal ground with no obstructions is supposedly 70 ohms. Location is anything but ideal, both ants in roughly the same place.

QTF?
 

[Steve - WB3HUZ]

In general, the less loss in your antenna system, the LESS broadbanded it will be. I'm guessing in your case, the new antenna system has less loss, and it's more in the clear, so it should be less broadbanded than the old one.

[WA1GFZ]

You raised the Q so bandpass is shorter.

[AB2EZ]

Todd

There are many (roughly equivalent) ways to understand why an antenna with less cable loss has a smaller SWR bandwidth.

One way is to think of a lossy cable as a power divider... with one accessible input and two outputs. One of the two outputs feeds the equivalent of a dummy load. The other of the two outputs feeds the dipole.

The SWR bandwidth, measured at the accessible input of the cable, is a blend between the SWR bandwidth of an ideal dipole, and the SWR bandwidth of a dummy load.

An ideal dummy load has essentially infinite SWR bandwidth. An ideal 3885 kHz resonant wire dipole (even with 10 gauge wire) has about 50kHz of SWR bandwidth.

If the cable has 3dB of loss, then the antenna's SWR bandwidth is a 50-50 blend of bandwidth of the dummy load and the bandwidth of the dipole. I.e., wider than a dipole's SWR bandwidth.

If the cable has 10dB of loss, then 90% of the power goes into the dummy load. But the SWR bandwidth looks close to that of an ideal dummy load.

If the cable has 0.5dB of loss, then 89% of the power goes into the dipole, and the SWR bandwidth is close to that of an ideal dipole.

To get more bandwidth out of a dipole, one has to use something like a "cage dipole", whose equivalent cross section is a much larger fraction of a wavelength (although still a small fraction) than that of a single wire.

Best regards
Stu

[Todd, KA1KAQ]

I see. So it's mainly a case of the previous ant system being so lossy (dummy load) that the transmitter didn't notice, didn't care, no large change in ant current meter across a wider range. Whereas a better ant with quality feedline would be much narrower and more likely to reflect power when out of tune. Makes perfect sense. I got hung up on the 'larger conductor = broader tuning range' tidbit from years past. Broader still doesn't equal 175 kHz.

It's not so much a case of being more in the clear this time Steve, mainly that it's insulated wire not hung up on anything, not bare copper laying across branches. The old ant literally burned through one branch which still dangles to this day. Probably when I was trying to load up on 160 with it. The new ant is up higher though, and can go up another 5-6 feet once I get the ends up more.

Looks like it's tuner time. Hmm....now, lessee....tune the transmitter into the BC dummy load first, set up tuner after. This prevents dancing to and fro between transmitter and tuner. Last time I used a tuner was.......91-92? 

Thanks for the lesson, guys.

[WA1GFZ]

you could get more bandwidth with additional conductors in a fan configuration. going up a couple sizes is not enough change in diameter to make a difference this is due to the long length..

[W1VD]

For a coax fed dipole one simple tuner arrangement is the T match - series C, L to ground, series C. For 80 meters, variable caps of a couple hundred pF are adequate. A roller inductor for the L is handy. One caution - this type of tuner can be adjusted for a good match at a number of different settings. The best tuning procedure is to start with both caps fully meshed and adjust the roller inductor for lowest possible reflected power. Then touch up the capacitors in conjunction with inductor for a perfect match (if desired)  keeping the caps as fully meshed as possible.

 

[WA1HZK]

Face it, you won! More current, more fire to the wire.

[Jim, W5JO]

Just remember what you did and when you get to FL repeat it with improvements.

[Todd, KA1KAQ]

I have to admit to being lazy/sloppy with respect to the old ant system. It tuned anywhere in the band without a lot of change, so it seemed rather cozy albeit wasteful. Surprising I didn't have RFI/TVI complaints from the neighbors.

One caution - this type of tuner can be adjusted for a good match at a number of different settings. The best tuning procedure is to start with both caps fully meshed and adjust the roller inductor for lowest possible reflected power. Then touch up the capacitors in conjunction with inductor for a perfect match (if desired)  keeping the caps as fully meshed as possible. 

Interesting, Jay. I was taught to start with the caps half meshed to save time/wear and tear on the tubes. The theory being that unless the sweet spot is right near the extreme, you end up doing a lot of extra tuning.

But past tuners have utilized tapped coils for band switching and two air variables. The one you described is the type of tuner I'll use with it. It's a Dentron 3KW unit, with the caps labeled for 'Trans' and 'Ant', roller inductor between. Strikes me that I could be cranking that roller for quite some time to find the perfect spot.

By number of different settings I assume you mean the final impedance match will be the same in several spots. If so, what determines the best? Least prone to flash over? I'll need to pull the lid to see where full-mesh occurs, just to be safe.

Static levels on 75/80 have been pretty high the last few nights. Heard quite a few 8 and 9 Landers on 3880 last night, coming in like locals. Robert W0VMC was booming in also.

[W1VD]

Start at max capacitance and adjust the L for a dip in VSWR. Think you'll find that the caps end up fairly close to full mesh when then adjusting all three elements for a perfect match. I use this type of tuner on each of my 160, 80 and 40 meter coax fed dipoles and it only takes 30 seconds or so of twiddling to get a perfect match. Since you have an rf ammeter leave it in line and have a look. An interesting test would be to then adjust the tuner using the smallest amount of C that will still give a perfect match and see what happens to the  rf current. Always wanted to run that test but haven't got around to it.  Let me know how you make out.       

[WA1GFZ]

Jay,
Your network is high pass. I wonder if you ever tried 2 inductors and a cap to make it low pass? fc

[W1VD]

In the C-L-C arrangement, higher values of C (primarily the C on the transmitter side) lower the circuit Q - a good thing to keep circulating currents and losses down. When matching two impedances that are fairly close to each other (50 ohm transmitter and coax fed dipole cut for the band in use) C vs. Q for 80 meters in rough numbers are as follows: 400 pF Q=2; 160 pF Q=5; 80 pF Q=10; 40 pF Q=20. Arbitrarily selecting a higher Q setting when a lower one will effect a match will lead to increased losses. Always go for the lowest Q setting...although little is probably gained once you're in the low single digit Qs.   

Same for an L-C-L arrangement except one would want to minimize the series inductors for lowest Q. 

[WA1GFZ]

I notice my Big tuner makes nice sparks when the Q is too high

[Jim KF2SY]

Todd,
Another thing you did not consider is the dieletric effects of using insulated wire instead of bare wire.  The antenna gurus generally say you should cut about 1.5 - 2% shorter for insulated wire.   Just another 2 cents...

Jim

[Steve - WB3HUZ]

For best efficiency, always use the minimum amount of inductance.

Start at max capacitance and adjust the L for a dip in VSWR. Think you'll find that the caps end up fairly close to full mesh when then adjusting all three elements for a perfect match. I use this type of tuner on each of my 160, 80 and 40 meter coax fed dipoles and it only takes 30 seconds or so of twiddling to get a perfect match. Since you have an rf ammeter leave it in line and have a look. An interesting test would be to then adjust the tuner using the smallest amount of C that will still give a perfect match and see what happens to the  rf current. Always wanted to run that test but haven't got around to it.  Let me know how you make out.