Something I do not understand on antenna "Cutting & Tuning"

[GreekMan]

so....a popular method of cutting your DIY for length is checking SWR with a meter.

But I have been told that this is not a good method, cos you are cutting for the least SWR and keep cutting and cutting till the antenna scores well, but becomes very inefficient in the end.

But this is not the method I am using. In between each shortening of the length I am checking SWR every 2 MHz or less, for a total 2x10MHz up and down (in the VHF).

If I understand it correctly this is not much different of what an antenna analyzer shows in its graph.

So..where I may be wrong?

[Tempstar]

Hi Greek Man,
 You throw out some interesting things, I'll see if I can help you out.
Tuning for an acceptable SWR is not the same as tuning for resonance, which is the most efficient. We tune for lowest standing wave to protect the radio from reflected power coming back down the feed line. It is quite difficult and usually very expensive to achieve an antenna and feed line match that is both resonant and presents a low SWR. Each connection, especially PL-259 connectors, presents a blip on the screen of a vector network analyzer that shows the many points in an antenna system that affect total efficiency. Your best bet at Ham frequencies and power levels will be to tune for lowest SWR, and do so with the feed line you will be using and the antenna, especially a mobile, mounted where it will normally be. I have seen many antennas tuned for SWR and then mounted somewhere totally different than where it was tuned. The mis-match is usually significant.
Hope this helps you some!

[ghrit]

GM-

Let me see if I can confuse you a bit more. 

If you tune the antenna itself for resonance, you'll check it at the feed point.  That will make the antenna itself as efficient as it's possible to be.  HOWEVER, as soon as you stick a feed line in, you are introducing some sort of reactance that will affect what the transceiver sees AND result in a mismatch of some type or another at the connection of the feed line to the transceiver.  That mismatch MAY alter what the transceiver wants to see, to the extent that damage could occur.  (Damage is unlikely with modern rigs, they have built in protective circuits that will cut back on power or simply shut down.)  Therefore, tuning for lowest SWR as the rig sees it is the best possible way of getting rf to the antenna.

Each and everything that is inserted into the "space" between the antenna and rig represents a measurable loss of signal strength and some reactance of one type or another.  Considering all the insertion losses and providing compensating reactance with a tuner (another loss, the tuner is an insertion itself.)  All else being equal, it makes no difference if the tuner is internal or external, they are losses which are acceptable from the standpoint of getting rf out of the shack.

I sure hope I got that right.  (I've put out incorrect info before on occasion.  If this is grossly off base, someone will correct me, I hope.)

73
ghrit

[GreekMan]

luckily I use standardized pieces of coax. but I will keep the point in mind for the future.

Each and everything that is inserted into the "space" between the antenna and rig represents a measurable loss of signal strength and some reactance of one type or another.

ouch that hurts.
meaning that coax losses affect SWR and return power will be greater at the antenna feedpoint than at the meter located at the radio.

[ghrit]

ouch that hurts.
meaning that coax losses affect SWR and return power will be greater at the antenna feedpoint than at the meter located at the radio.

Not quite.  Assuming perfect resonance at the antenna feed point, SWR will be an absolute minimum at the feed point for the singular frequency it is tuned for.  Then, each item inserted is a reflection spot, so yes, SWR goes up with insertions when measured at the xceiver end of the feed line.  If the antenna itself has losses, well, they all add up.  Then, it's up to the tuner to fool the radio into thinking it is dumping power into the atmosphere, even if the losses add up to a significant reduction in output power at the antenna.

The idea of using a standard coax is the way to go, the moreso if it matches what the xceiver wants to see.  If your feed line is, for example, 75 ohm coax, you'll need some other trickery for matching.  (Like a balun in the right place.  I think, but do NOT know, that most tuners have that capability on board.)

[Tempstar]

Dead on Ghrit! And yes, most modern tuners have no problem with 75 ohm feedline. I use 1604 Belden video cable to my Chameleon and it gets a better match than with 8X.

[spacecase0]

I have seen an old way of tuning that seems very accurate,
you make a wire loop that sits on the antenna, it is connected to a small light bulb.
you slide it along the antenna till you find the null point where the light is dimmest, and you cut the wire at that point.

[Tempstar]

Edit: 1694 Belden cable. I hate typing in the dark.

[DragoSapien]

Im here!!!!!!  Just reading and learning what you all are teaching.

[BTPost]

The way “The Big Boys” do this is to put a Impedance Bridge on the feedline at Tx Antenna Port, and the tune the antenna by observing the resonance of the whole Antenna System... When I was designing and installing multi Band HF Marine Coast Station Antenna Systems, I use my Cushman CE5A with Tracking Generator w/Spectrum Analyzer that had an Impedance Bridge that connected the Signal Generator & Spectrum Analyzer on opposite Ports and the Antenna in Question and a 50 Ohm Load on the other two Ports... I could see the exact Resonance Frequency Point and then watch how it changed when an adjustment was made, then I could calculate how much adjustment would be needed to get the resonance point where it needed to be for that band... On Trapped Antennas, you do the Higher Frequency Band, first, and work your way down for each lower frequency band....  I designed and built many muti-KiloWatt, Multi Band Antenna Systems, over the years...