HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on December 29, 2020, 1931 UTC
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50 ohms is a compromise for loss and power handling. 30 ohms is the ideal impedance for power handling, 77 ohms for loss, hence the use of 75 ohm coax for television and other applications. For receive only setups, 75 ohm RG-6 is often your best bet for coax cable. It's inexpensive, low loss, easy to find and work with. Even for many transmitting applications, it works OK.
https://www.microwaves101.com/encyclopedias/why-fifty-ohms
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Yep... I've used RG-59 and RG-6 coax for years from VHF TV, FM RX'ing, HF longwires, and scanners. Heck, come to think of it, I'm using it for TX'ing from the Talking House / I Am Radio AM transmitter to the ATU. That's all F-59, 75 Ohm coax standard.
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Been using RG-6 for years here for RX and TX (100 watts or less).
Stick with the name brand stuff... it costs a few pennies more but
is far superior to the no-name junk with seemingly ultra thin copper
cladding. The thin copper may be OK for TV use but don't feel
comfortable with it myself for long runs at LW and MW.
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Lots of contractor-grade RG-6 in use here from LW through UHF. It is affordable, it works for my purposes, and it is readily available at Home Depot, Lowe’s, etc. T&B-style RG-6 snap connectors and basic compression tools are relatively affordable on eBay, too.
I do have RG-8/213 and other 50-ohms coax types in use, but I could just as easily done most of those runs using RG-6 with little to no appreciable difference.
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Hey practitioners. How about the mismatch between the 75 ohm cable and the 50 ohm optimized baloons? Does this have any noticeable deterioration for our receiving antenna installations? It seems that none of us changes the number of turns of the transformer, keeping, of course, its gear ratio.
Is the signal loss worthwhile?
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Hey practitioners. How about the mismatch between the 75 ohm cable and the 50 ohm optimized baloons? Does this have any noticeable deterioration for our receiving antenna installations? It seems that none of us changes the number of turns of the transformer, keeping, of course, its gear ratio.
Is the signal loss worthwhile?
A good question to ask first is... what's the actual impedance of your antenna?
A resonant half wave dipole has an impedance of 73 ohms for example, so 75 ohm coax is going to be a better match than 50 ohm. And that 73 ohms is only valid for a narrow frequency range, and assumes your antenna is in free space. It's not, it's over the ground, probably not very high, so the impedance is not going to be 73 ohms. It's quite possibly not resonant (zero reactance) anyway :)
If you're using a random length antenna (aka "longwire"), the impedance is going to vary dramatically over your listening range, so the impedance of your coax cable likely not going to matter much.
If you have a T2FD or similar antenna, you'd want the transformer turns ratio to be correct for the coax impedance. But even there or for other antennas a mismatch of 50 vs 75 ohms is not going to be a deal breaker for a receive only antenna.
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Thanks Chris for the comments.
The SWR chart for my Sky Loop looks like this:
(https://i.imgur.com/JNpaTjb.png)
We see a very irregular course of the SWR, which is telling about large changes in the wave resistance as a function of frequency. In view of this, the mismatch between the 50 ohm transformer and the 75 ohm cable seems to be irrelevant to the reception.
I would prefer my SL to have a SWR chart as flat as your T2FD, but a level below 2.5 in a large part of the HF band is satisfactory for me. I still think that the SL is better than the Longwire, Windom, open dipole, vertical whip I have used before.
I found the description of the Sky Loop matching box to transmit on the 160m band. I think it makes sense to improve the reception on the "free radio" bands that interest us!
Chris wrote:
I see similar SWR excursions on my Sky Loop, and like yours it works great. Another example of why worrying about SWR with receive only antennas is pointless ;D
Thanks Chris, you calm my nerves! ;)
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I see similar SWR excursions on my Sky Loop, and like yours it works great. Another example of why worrying about SWR with receive only antennas is pointless ;D
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What matters for receiving is S/N, not pure S level.
We want low noise antennas, or if the antenna is just standard, we want to place it outdoor where the signal is higher and where the noise is lower.
Then, for a distant antenna, we need a 'down' line. That line can carry the house noise to the antenna (common mode or outside of the coax), and that noise may be carried backwards to the shack in differential mode or inside the coax.
Also, the tuning accessories can be not well shielded and can pick up noise directly. Then, if there is RFI noise in the shack, the ATU must really tune the antenna to get the more of signal coming from the backyard, above the fixed noise level from the shack.
If a coax line can be buried all the way to the antenna, and grounded both at the antenna bottom and house entry, that's the best, but the antenna impedance should be not too far from the coax impedance on the bands of interest. Then the antenna should be also not too bad from itself about the neigborhood noise (so verticals are not very good in a typical suburban location). My own attempts with coax lines are not good so far, for practical reasons.
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My coax lines are buried underground (see my previous posts with more details) and grounded at several points - at a minimum at each end of the buried portion. Some also have common mode chokes to reduce RFI currents flowing on the shields.
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What matters for receiving is S/N, not pure S level.
^Many times THIS.
Most modern receivers have incredible gain potential. It is called the volume knob, AF control, etc. ;)
There is a reason I keep my loop-on-ground. It has horrid loss, but it can have better SNR in certain instances. Oftentimes I can have an easy signal copy yet the signal needle is under S1 or even barely moving at all.
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The mismatch between 50 ohm and 75 ohm coax will create a SWR of 1.5/1.
Some hams will obsess over this in search of the perfect 1:1 match.
However, a SWR of 1.5/1 works out to a reflected power of only 4%.
See the ARRL Antenna Book Chapter 23, the section on Standing Waves,
they got a nice graph in there showing the how much power is reflected
for a given forward power and SWR.
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The mismatch between 50 ohm and 75 ohm coax will create a SWR of 1.5/1.
Some hams will obsess over this in search of the perfect 1:1 match.
However, a SWR of 1.5/1 works out to a reflected power of only 4%.
See the ARRL Antenna Book Chapter 23, the section on Standing Waves,
they got a nice graph in there showing the how much power is reflected
for a given forward power and SWR.
The amount of radiated power wasted is not the primary concern when dealing with VSWR. I will apologize ahead of time for derailing(at least temporarily) this thread ;D
In the commercial world, best reliability comes with systems designed for low VWSR. High VSWR is not only wasteful of TPO, but also causes hotspots in the line. When you have perhaps 20% overhead on line capacity because the next size up is prohibitively expensive, low VSWR needs to be maintained to keep the line safe. We are also in the era of solid state transmitters, which don't like VSWR. Even commercial AM's will only operate at full power up to 1.5:1 before folding back. Typically in commercial FM service, the antenna match at the output of the transmitter is under 1.1:1. High VSWR in TV service causes problems with intermodulation and spurious emission in the power amplifiers, so they tend to run lower, typically under 1.05:1 to maintain mask clearance. The gear I build is designed with enough voltage rating to handle full power operation at up to 1.5:1. We are on the bleeding edge of technology, and until we have transistors capable of 2KV or better Vds ratings, this will be the ceiling.
All of this, of course, is completely unrelated to receive antenna performance, where major shifts in impedance or standing wave will have little affect on the ability to receive a signal.
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My response was directed more at the hobby world and not the commercial world.
Even so, can't recall ever seeing a SWR meter in a land-mobile or avionics repair shop.
The Bird 43 was (is) the go to instrument for checking transmitter/antenna power
output performance. For example, if a mobile unit rated 40 watts is putting out
40 watts with, say, a couple of watts reflected... that unit is going to go out the door.
(The SWR would work out to about 1.6:1.)
Nobody is going to tinker with it for an hour or two, much less bill the customer,
trying to get the reflected power down to zero.
In the ham world, if you want to or feel the need to address the 50 ohm to 75 ohm impedance mismatch,
you can always build a 1/12 wave matching section.
btw... there just so happens to be an article in this month's QST regarding the construction
of a 50 to 75 ohm matching transformer.
Of course, for receive only application, all this is of less concern.
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As for using 75-ohm coax in a 50-ohm system, if tuning for a specific frequency or even a specific narrow band, using a half wave multiple of 75-ohm coax times its velocity factor generally works as well.
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50 ohms is a compromise for loss and power handling. 30 ohms is the ideal impedance for power handling, 77 ohms for loss, hence the use of 75 ohm coax for television and other applications. For receive only setups, 75 ohm RG-6 is often your best bet for coax cable. It's inexpensive, low loss, easy to find and work with. Even for many transmitting applications, it works OK.
https://www.microwaves101.com/encyclopedias/why-fifty-ohms
I learn something new everyday, thanks 😊